Ok, so it sounds like you actually have two problems.
- How do you deal with the little things
- When do you expand
I'm going to address the second one first.
Don't expand if you're still dealing with harassment and large armies. Expanding is very much BW style play and largely is designed around the concept: Expand when I have Map Control. If you don't have Map Control and you try to expand, what's going to happen is that your opponent is going to shut you down with some small task force (like 12 Reapers or 8 Muta). You're going to respond with your whole army and he's going to pull away with little or no losses. So if you're losing expansions to harassment, chances are you just don't have the map control to expand.
Next you talk about how you're losing larger armies (40% to micro). Almost everything your army does requires 0 attention from you, except combat. If you're engaging in a situation where you will lose (split army) your response should be retreat to somewhere you can win. If this means sacrificing an expansion, sacrifice it. If you're walking into an unscouted position, retreat. Scouting is a far more valuable use of your time in these situations than micro is.
Now, if you're losing because your Zealots are in the back and your Stalkers aren't blinking, and you're running into storms, then the problem you're really having is how you macro. You should be building and organizing your units as part of your macro. If you're building your Zealots behind your Stalkers and leaving them there, that is bad macro.
Finally, if you are engaging a scouted opponent and thinking that you should win, then give over most of your attention to this battle. Perhaps build things via hot keys only, but don't move your screen for more than a second. Big battles require a lot of attention, and you should give it to them.
You also mentioned harassment in a couple of contexts so let me give you some good advice. Photon cannons are not designed to stop harassment, they're designed to delay it. It is far better that his Mutalisks spend the time to kill 2 photon cannons, than you build the 8 photon cannons to stop this raid. Use defensive structures as a delaying tactic.
+1 for the dream analogy!
Like dreams, there is a good explanation of why you are living your game in slow motion, there is always one thing slowing down everything else. In the case of dreams, it is your brain, that actually dreams faster than your perception. That's why you seem to be slow, because the dream is actually faster than what you perceive.
For StarCraft, this is another problem. I discovered by studying replays that faster players remove one very large part of the time you may take in your game: controlling the army.
As an unexperienced player, you might not be familiar with unit balance. So when sending your 12 zerglings againts those 4 zealots, you don't yet know the outcome. I'm sure you expect some victory, but can you trust this expectation enough to actually leave your zerglings do the work without you?
I often find myself microing too much when dealing with large overpowered army. Yeah, I have 24 zerglings and they meet 1 zealot. Really, no need to micro them, just return and take care of your base!
Your problem may be the same...
Best Answer
Select Quickly
First thing to do is remember how to change what you have selected quickly, so that you don't spend any extra time trying to get a box around just the right guys to set to your group. You might already know all of these but:
Shift-Click and Ctrl-Click work on portraits too. And these work together: "Remove all units of that type from my group" is Shift-Ctrl-Portrait Click.
Group By Role
You will always want to group by units that you will be giving the same orders to, so you don't have to alter the group any after you select it. I find my groups work out like this:
I sometimes group by move speed as well, because sometimes you just need any troops here right now!
Almost is good enough
Finally, remember that its ok to miss a couple guys because new guys were built, just go pick them up immediately after. (I find Shift-Box to be the most useful for adding the new guys to the group.) All the guys in a group won't arrive at the same time anyway, and having MOST of the group start moving toward where they are needed is better than none. Just update your groups before you move away from your base, or whenever you give them new orders, but AFTER the order.