I think you are right in assuming that "everyone pulls their own weight" is not very helpful or realistic. However, everyone can pull different weight.
In my group I'm the host, which means I clean up after my friends. In return, I don't have to travel on game night - an even trade as far as I'm concerned.
I think when you are the "one in charge", you should delegate as much as is practical and enjoyable. Ask your friends to use their strengths to make the game better. Maybe one is a skilled cartographer - have him make a map and make it your challenge to incorporate it into play. Give another a particular faction and have her detail their motivations and plans and stat out some NPCs. Her reward will be seeing you breathe life into her creations, and you'll have both inspiration and a reduced workload. I really think the idea of the all-powerful GM who has to attend to every detail in isolation is unnecessary.
Maybe that isn't practical - all your friends are busy, talentless, lazy, whatever. In this case you can at least level with them, explain how much effort you put into the game, and ask for their ideas about how to make things easier or more equitable. Maybe they can split your share of the pizza as thanks. Maybe a candid discussion will lead them to decide they aren't so lazy, talentless, or busy after all!
I know you're not a native English speaker, but in many circles of general nerd culture in America and on the Internet, there is a phrase for what you're doing right now: your spaghetti is falling out of your pockets.
In all seriousness, though, you seem very anxious about your situation and you should take a step back and relax. You say you're good at improv, but it looks like you're already sweating bullets because your party got sidetracked instead of getting to the quest you wanted to take them on.
I'm going to put my direct answer to your question first and some general advice afterwards.
Your players probably won't miss their characters. Why? Well, you generated them, and even wrote up their backstories. These characters were never made by the players playing them, and while they might have grown attached over a few moments, I find it extremely unlikely that they'll shed tears over a party that lasted for four sessions... that they didn't even make themselves. Even if the characters are extremely cool and interesting, that element of personal attachment just isn't there, which allows the players to truly say that they helped create an exciting story with interesting characters. This obviously varies from person to person, but again, I find it very unlikely that your players would have grown very attached in this case.
Go out with a bang. You know that amazing epic encounter you were saving for the climax of the plot arc? Yeah, run it now. You'll have to make some tweaks because you obviously aren't quite there yet, but get them there as fast as possible. Feel free to kill off PCs or even have a TPK at this point; memorable deaths are often much better than "and then they lived happily ever after."
Here's a few things to keep in mind when starting your next campaign:
Your content will come to light eventually, and it will be good. The quests you've designed will always find a way to come forward. Even if your plot arc is entirely ruined by something the players did, you will be able to recycle the content you made but never ended up playing, and I encourage you to do so for your new campaign. The only things that are truly lost are "hard" materials, like NPC stat sheets, etc.
You're in control, and therefore, you set the tone. Sometimes, it is best to take a page out of Gygax's book; after all, this is your campaign, and you put a lot of effort into it. Obviously you shouldn't take the entire preface from the AD&D DM's Guide to heart, but there is a point where a DM should draw a line in order for there to be some kind of structure, assuming you want your campaign to go anywhere. If your players are goofing around and killing NPCs for no reason, or making light of important people in-character, then they should be ICly punished for it; reprimanded for insulting a nobleman, pursued for attacking innocents, etc. It is also very possible to play a serious game in character and laugh until you're blue in the face out of character. This frequently occurs in the Dark Heresy games that I've played and ran.
You had better get used to murdering your darlings. This is a phrase commonly used amongst writers and creative designers everywhere in the U.S. The phrase means that you'll have to scrap ideas frequently, including ones that you really, really liked, so you had better get used to it. The saying is intended for use in the writing, film, video game, and other industries where a publisher or producer oversees your work, constantly telling you what can and can't make it to the final product based on time and expenses. However, it works just as well for when your ideas can't make it to the game because your players did something insane. And, on a related note...
Plan less. I don't know how much effort you're putting into writing everything ahead of time now, but you might want to ease up on that. From what you're telling me about your role-playing experience, it seems like you've been playing in a "safe" and slow environment where you rarely, if ever, have to scrap or re-do material. This happens literally all the time in regular tabletop RPGs, thanks to the insanely unpredictable nature of 4-5 different people working together. It will save you a lot of anguish if you lay out a basic outline of what's going to happen and then add the details once you're sure the players will be arriving there next session, or maybe two sessions later.
It seems like you've learned a lot already OP, which is great, but scrapping a campaign after 4 sessions (and while your players are all enjoying it) is something you should really avoid. If everyone else is having fun, consider either shaking things up a bit and changing your own notes, or coming up with a way to set them back on track, which doesn't always need to feel contrived or railroad-y.
EDIT: Well, now that SevenSidedDie has made that edit to your post, there are a couple of details that I didn't quite catch before, no offense. Since you said you have a month between each session, it seems like you might be over-planning because you have a lot of time between sessions. Heck, you might even consider having more frequent sessions, if you can't stop yourself from overthinking it in the intervening months. If in-person is not an option, use Skype and/or Roll20.
Best Answer
The new GM is absolutely in charge of the new game. If you want to talk with them privately outside of the game or if they come to you for advice, great, but at gametime you need to play your character and live within the confines of the game as it is. If you've got any tips for getting the new GM over the learning curve, offer to share them.
For coping mechanisms, try these ideas:
Mechanics Assistant GM If you want to offer your services as a game-time rules resource, that might work. You could speed up the game for everyone by providing rulings when called on to do so if it's in your favorite system. Or you could man the rule books and be the designated lookup guy. It taps your strengths and helps the game as a whole. I wouldn't do that without checking with your GM first, though.
Enjoy the Opportunity Here's your chance to be a model player and show the other players how you would like players to act at your table. What did your players do that annoyed you? Don't do that. Pay attention. Ask clarifying questions without sinking into game-stalling details. Remember how you didn't like your players checking their cell phones every 5 minutes? Don't be that player.
Quick Notes Are Your Friends If you get ideas during the game, scribble them down and deal with them later. If you have ideas for a series of How To GM a Game articles, note them and get back to the game. After the game, feel free to draft a setting book for the next game you want to run, but it shouldn't come up during play of your current game.
Know When to Walk Away If your ideas start getting in the way of the current game, like if you can't pay attention to the game because you're scribbling future game ideas, maybe playing isn't where you should be spending your time. Some people have a hard time "only" playing after GMing. They're different skillsets, and the focus on different things. If you think that the current game is a waste of time, you've got 3 choices: 1. Try to improve it by talking to your GM. 2. Find a different game. Sounds like this one isn't a viable option for you. 3. Stop playing and let yourself create your next game in peace.
For your next game, maybe one-shot adventures will work better for you. Less prep work, and a decent dungeon crawl can last a few game sessions.
An aside: I've managed to avoid GM burnout by giving my players a bigger piece of the plot pie. Let them take control of what happens next a little more. But that's another topic entirely.