Until I started to paint, I didn't think this would be an issue. When you're satisfied that you've painted what you intended, then the painting is finished, right? Not really. From the beginning, I've found myself going back and touching up, fixing areas I wasn't satisfied with. Or adding detail I thought the painting needed. Or changing a shade or adding a tone. At any rate, it seemed that I'd never finish anything. Then I decided to sign the painting. That would finish it. Not really. I found myself going back and changing things after I signed it. I recently heard this is not that unusual. Picasso would not feel at all uneasy to change something on a signed painting even years later, even after he sold it.
There is a time when you wonder if you're finished. I usually wait for a few days, checking the painting out daily to see if it 'needs' something changed. At some point, I just sign it and that's usually the end of it. I've come to grips with the feeling that I won't be happy with each and every painting I do, even after going back day after day and making changes.
At one point early on when I sarted, I decided to take all the paintings I didn't feel comfortable showing to anyone and cut them up into two inch squares. I took these and made a collage which did give me some satisfaction with an image which was more pleasing than any of the ones I cut up.