The reason bots only select a limited set of heroes is because the AI hasn't been scripted for every hero in the game yet. Assuming you pick before them, the bots will then try to counterpick any heroes you or your allies have.
You can also 'suggest' heroes which will cause allied bots to choose those heroes, if AI exists for them.
As far as I can tell, the only AI scripted heroes currently are:
- Axe
- Bane
- Bloodseeker
- Bounty Hunter
- Bristleback
- Chaos Knight
- Crystal Maiden
- Dazzle
- Death Prophet
- Dragon Knight
- Drow
- Earthshaker
- Jakiro
- Juggernaut
- Kunkka
- Lich
- Lina
- Lion
- Luna
- Necrophos
- Omniknight
- Oracle
- Pudge
- Razor
- Riki
- Sand King
- Shadow Fiend
- Skywrath Mage
- Sniper
- Sven
- Tidehunter
- Tiny
- Vengeful Spirit
- Viper
- Warlock
- Windranger
- Witch Doctor
- Wraith King
- Zeus
Valve periodically adds new bots in their weekly updates, but as of late they have been focusing primarily on AI enhancements instead of adding more heroes.
Before getting to work, my first advice will be: Play carry. Train and learn to play a hard carry that needs safe farm to be efficient. While you'll be playing, you will notice immediatly what your support needs to do in order to protect you. As far as I remember, being the most experienced player in my party, I always played carry. Needless to say that when I switched to support I was able to protect the lane and offer safe farm for my carry as I had myself been carry for a very long time and I knew as a carry what I wanted my support to do. Now I understand that this require a lot of time but this is the best way to learn to support, I can assure you. For the moment I can offer you some tips based on my exeperience of the game, starting with....
How to react depending on the enemy lane
If you are familiar with all the heros, your first reflex is to foresee (depeding on the hero picked) what type of lane the enemy team will run and adapt your behaviour. If for any reason you feel you need more help in the lane, do not hesitate to bring one more hero and run a defensive trilane.
If you are laning against :
- A passive solo offlaner (furion, elder titan for example) : Zone him out as soon as possible (even at first wave). You must zone him out of xp range. Do not hesitate to trade hit with him. Those offlaner dont have a great offensive power, if you have enough regen (tangos) you can force them out of the lane and deny a lot of xp by stacking and pulling (see last section).
- An agressive solo offlaner (darkseer, weaver, clockwerk, windrunner). Usually zone him out with one more hero at the very begining, then stack and pull. Those hero have great killing potential even in 1V2 situation. They might get over confident and put themselves out of position which allow an easy kill (especially with Crystal Maiden when you have a strong slow and a great stun).
- An agressive dual lane : this often ends up as a 2v2 situation. In this scenario the best you can do is sit in lane with your carry and save regen for him (pull him tango). If the 2 enemy hero are strong laner just stay passive, deny creeps as much as possible an try to get back the lane equilibrium close to your tower. It's the responsability of your carry to not put himself in danger aswell. As soon as possible request a gank from your mid. If the 2 heros took a level advantage and if they both get killed you will gain that advantage back in seconds.
- An agressive trilane. Play as defensive as possible. They brought 3 heros here which means they are losing in xp and gold. You can take advantage of it by not dying. Winning versus an agressive trilane is really as simple as not dying. They will shortly fall behind in levels and either abandon the lane or get killed by your mid when mid games kicks in.
In any case : don't forget to buy sentries/dust if you are laning against an invisible hero !
Items you can concider buying
Those item will bring some lane sustainability and are affordable for a support :
- Ring of basilius : mana regen + armor aura (don't forget to turn it off in the laning stage so the creeps dont benefit from the armor and the lane wont push toward your enemy tower : a place where its more dangerous to farm for your carry)
- A headdress of rejuvenation : Good stats and AOE health regen.
- 2 or 3 sets of tango : you can share them or trade hit with the enemy without worrying since you have plenty of regen
- Remember that you can share ring of health/regen, if you are planing on building tranquil boots, you can temporary lend the ring of regen to your carry.
- Wards (obviously) : the following ward's spots are often overlooked, however they are great to provide lane safety for your carry :
1) To protect jungle on the dire side
2) This wards allow you to see if a gank is coming from the tower and cover the river entrance (dire side)
3) This one protect the jungle on the radiant side and even allow you to see the rune
4) This one allow you to see an upcoming gank from the enemy tower as well as the river entrance (if the enemy mid come with a rune for example)
Use them in the following order : When you buy your first set of wards, give one to the offlaner and put the other in the jungle (1 or 3). If you are on dire side and your mid need rune control, concider warding the rune spot at the begining instead of the nearby jungle. Once you can afford your second set of wards, ward the rune and ward the spot that cover the tower (2 or 4). Usually the time where your recieve the second set of wards match the timing where enemies start rotating to your safelane in order to gank your carry. If you can see them TP to their tower you can avoid unnecessary death or foresee a push, plus it allows you to see an enemy venturing too far in your lane and getting out of position : allowing an easy kill.
Asserting lane dominance
It might sound silly but : play agressive.. from the start. If people don't fear your Crystal Maiden, Frosbite them from the begining and hit them. people underestimate how much magical damage you can dish at low level. Dont worry about wasting your mana, you should get arcane aura at level 2 or 3 anyway and level it up at least twice. Let your carry last hit while you frost bite every enemy that come close to him, it works like a charm.
For more information
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Best Answer
I understand your feeling because in my almost 10 years of DotA I've been there aswell. As you state there is no best response to your problem than practicing. In DotA you never stop learning and even though you feel you are no longer getting better, you are still getting more and more experienced with every game and in the long run you will be able to overcome this problem, given that you want to get better of course.
Now to answer your problem direclty there are several things you can do.
1) Read guides
You've been playing for over a year which means you have the basics of the game's mechanics. You can now start reading complete and full guides of heroes that you have trouble understanding. Every hero in the game has one or several guide on the official dota site. Those guides often describe how to play the hero and how to behave in the different common situation that you'll be facing in a DotA game. The guides also list their strength and weakness and how to overcome them. Some might be outdated but that doesn't mean the advice you'll find in them are outdated. Start reading !
2) Learn to recognise a threat
Playing a squishy hero is hard because you often become an easy target for hero with lots of burst damage. If you start playing too cautiously you'll fall behind in XP and gold and fail anyway. The first thing you have to do when playing a squishy hero is to recognise on the enemy team which hero is a threat why and when.
By when I mean when in the game will this hero try to kill me ?
The enemy mid is not likely to come early in your lane however the support can rotate and try to gank you.
By why I mean what ability/kind of damage makes this hero a threat
If I can go invisible, can he detect me ? Does he deal magical/physical damage ? Is he a good chaser ? Does he have a lot of disable ? etc.
Once you've answered those questions you can play accordingly and aim for a defensive item suited for the kind of threat you are facing.
This lead to...
3) Itemization and skill built
You have several ways to "toughen" you up in the early game. Here's a list of common items you can get to help you survive and sustain if you are being harassed :
Another thing that is often overlooked is that tankiness is not the only way to go when you are squishy. Mobility is the key :
When picking your skill take a few second to decide if you want to :
Those decision impact the way you can play and let you be more agressive if done right.
4) Map awareness and positionning
Last but not least : knowing that you are safe or not is important. Having wards placed at strategic path that enemy will most likely take if they try to gank/kill you is vital. If you know where they are, you know how you can play. If you can't see them : play safe. However if you know where they are you can either keep on farming, go get a rune without risk, in other word play as you wish since you know you are not in danger.
When it comes to teamfight the key with squishy heroes is positionning. You are squishy, you are a target => don't put yourself in danger. By that I mean don't go first, you have (or at least should have) some kind of initiation in your team. Let the initiator (Tidehunter, Earth Shaker, Void, SandKing, etc.) do his job. Let the heroes in your team that control the teamfight (Brewmaster, Clockwerk, Doom, Axe, etc.) do their jobs and position yourself accordingly. They are supposed to be the target so let them be, stay behind and when your hero comes into play (either by using your skill if you are a Lina or by atacking if you are a sniper) you can follow in the melee and start doing work. This is why Blink Dagger/Force Staff are so important : they allow you to jump in and out of fight and reposition quickly when needed.
In a nutshell
If you want to get better, you better start reading. Practice makes perfect but theorical knowledge helps a lot. Playing DotA is all about decision making, force yourself to analyse the game and ask yourself the question about how is the game developping and how you should play according to that. Finally if someone is bullying you => mute them. Let them say one bad thing to you and let that be it. You can't be reported for being bad however you can report them for communication abuse. Being bullied and letting someone take your moral down is an assured defeat so try to stay in a positive state of mind and mute toxic players.