I think it's all about motivation, because even if the end product won't be amazing, a 7/10 product is better than an unfinished 10/10 one
So yeah, it's all about motivation in my opinion, and it's hard to motivate yourself. You'll have a really cool idea, start doing it, but then the things get more repetitive, more boring, the "bad" part comes around, this is a very common part of any project and is generally called "The Wall". Almost ALL projects you'll ever work on have a Wall, an area of the project where you're just not willing to work anymore, you're demotivated, bored or feel like it's just not worth it anymore, when the only motivation for a project is yourself, it's very easy to stop working on your project at this stage. So how do you get past this motivational blockade?
The way that works best for me is to have others rely on you finishing this project. When you're in a job, you will still hit the wall but you're gonna keep on going cause others rely on you, there are negative consequences for just quitting so you can't just quit. For your art, maybe start taking requests? Someone goes "I want X", you got to give him/her X, if you don't you'll disappoint them. If you think your art is good enough, do commissions, if you don't do a commission you'll never be able to work as an artist, so even if you are the wall, even if you feel the art isn't the best, you HAVE to do it, there's no going out of it and, guess what, you will do it and it will be pretty good. Working in a team is my preferred way of getting motivated, when I'm in a team I generally get really excited about the project, even if I'm the one who motivates everyone I still feel like I should and can do this. To see other people eager to work on a project makes me eager to work on it, even if we hit a wall.
So yeah, it's all about motivation, you can't trust yourself for sole motivation of a project, try get external motivation somewhere.