1) Keep it fun. Sometimes you start working on something and a week later you realize it's not as great as you expected it to be. Maybe you are working on some game and you can see it won't be as fun to play as you thought, etc. It's fine, it's ok to stop working on it. Forcing yourself to work on it won't produce any good.
2) Start with well defined, small goals. Define v1.0 which would contain only the most critical features, ship it (even if it means simply tagging on git). There's something hugely motivating about completing things, no matter how simple they are.
EDIT. One more thing I'd add, when working on a side project, don't start thinking about how everyone will be using, how many thousands of github stars it will have, whatever. It doesn't even have to be actually useful, as long as you have good time and learn something on the way. I've worked on a few projects that I'd say are totally useless (such as https://github.com/GedRap/xs-vm ) but at the same time I see them as successful because a) had great time b) learned something new, even if it's some small details about a library or a language.
1) Usually getting something finished is a boring slog. You get the (you think...) brilliant idea phase when you're all fired up, then you get "this is really boring" phase when you're working your way through a thicket of problems, squashing bugs, finding new bugs, thinking you should refactor (you shouldn't, usually) and completion seems like an ever-receding goal.
2) Break big goals into smaller goals. Repeat recursively until done. Then do it some more, because you missed a few things. Sweat the small stuff, because when you glue it all back together and you've worked through phase 1 properly, the big stuff just works. (Or near offer, anyway.)
Keep the cognitive load low: small microprojects and problems, one at a time.
It's not unusual to have problems with phase 1. But it's also the difference between being a professional and being a hobbyist.
If you just keep starting new projects when you hit phase 1, you don't ship.
>>> You get the (you think...) brilliant idea phase when you're all fired up, then you get "this is really boring" phase.
In my experience, there are 2 kinds of 'really boring' phase. One kind, as you mention it, is the less exciting part of the work, be it writing docs, or some edge case bugs, etc. The other kind is when you discover that you largely underestimated the complexity, or saw some major flaws in the initial idea. Like I've mentioned in my previous comment, maybe you start working on a game and find out some major flaws in the gameplay and can't see it being a fun game to play anymore. While it's not always obvious which case you are facing, in the later case, in my opinion, it's fine to halt the project. If you don't believe in it, nothing good will come out of it.
>>> It's not unusual to have problems with phase 1. But it's also the difference between being a professional and being a hobbyist.
I guess it depends on what's the purpose of the side projects. If it's something you try to make a business of (passive income kind of thing), then yeah you totally should get a bit of extra will power to get it shipped. But if it's something that is done just for fun, then I wouldn't sweat it that much.
Problem - I can't draw. I have no idea what I'm doing. I am completely terrified of making some kind of horrible messy fuck-up non-art and...
To keep it short - it's all fine. I'm not Durer, and I sketched something that didn't look much like rope. But it was way better than I expected.
The takeaway: there's a kind of pain barrier with a lot of projects. You think "This is not working, it's not going to work, I'm totally wasting my time on it", and all kinds of other critical thoughts.
Then you keep at it, and often - usually - it all works out.
The one exception I've found is that if projects for other people feel stuck, there's often a good external reason, and it's possible that you're genuinely wasting your time on them.
But if it's a solo project and it's not working, you don't have to force that iteration to work. But if you keep pushing and iterating and changing, something good falls out. IME the intense frustration is often the sign you're on the way to a breakthrough.