Maybe you need to step back a little and do some analysis on yourself. You say you don't know why you stop working on things....why not? When you put a project away, do you already secretly know you're not going to want to come back to it? Do you put it away with regret that you have to stop, or are you glad to stop and can't wait to get it out of your sight?
Have you EVER finished a quilting project? How did you feel when you finished it? Were you glad to be done? Did you feel proud of your creation? Did you feel a little sad that the project was over? Did you hate to see it leave you? Maybe there's something about finishing a quilt and/or giving it away that makes you melancholy and you're subconsciously avoiding that feeling by not finishing them?
Do you work primarily from kits and patterns or do you do your own designing? Maybe the design phase is really what's fun for you and actually making the item isn't as thrilling. Maybe you're more of a designer than a maker? (I'm sort of like that.)
So there's some homework for you, LOL. YOU hold all the answers here, you just have to stop and observe yourself like an outsider would to figure it out.
But here's some good news - it's TOTALLY OK to just get rid of all your UFO's! Donate them somewhere; someone else will be happy to finish them and you'll be rid of these projects that seem to be weighing on you. And if you start to pull them out to donate and you find some that you just can't bring yourself to get rid of, maybe those are the ones you should start working on again; something about those projects in particular is calling to you.
Even if you're not ready to let go of any, try going through them as if you WERE going to get rid of all but one, and see which ONE is the most special or appealing to you and start with that one as your next project.
Good luck!