The best way to learn is start with smaller projects. Start with things you can finish in a day, then maybe in a week.
A lot of projects fail because the creator did not realize the scope of the project - that is, they didn't realize how much work it would be.
So, obvious things like "how much money do I have", but also things like "how much time do I have" or "do I have the right skills".
There's a lot written about perfectionism and I'm not going to give you a cure all or in depth therapy, but here's some small advice that might help.
No, seriously, go make some garbage.
Don't even THINK about that project that you've been passionately working on in the back of your mind since you were fifteen. That is SACRED, and you KNOW you want it to be perfect.
So don't work on it yet.
So come up with a new idea. Something small and interesting, but not something that sets your heart on fire, because you gotta practice some detachment.
You don't even have to show it to anyone if you want! (This is YOUR garbage. You can do whatever you like!!!)
The reason deadlines are so good is that they force you to practice letting go.
But whatever you make is not intrinsically tied to your worth as a person or as an artist. I promise.
Your project is a thing you made. It's not YOU.
Yeah, most likely, your audience doesn't think it's garbage.
And even if they do? People LOVE garbage.
I have garbage things I love and I bet you do too. Something being garbage doesn't make it unlovable.