This is one of the hardest things when working with physical medium. In digital painting one can erase easily, work on several layers, mistakes can be corrected in a flash. When painting on canvas or paper- you loose the support you have well prepared, and you can loose the great picture you've started but then went to continue, and then it went all bad. And when you try to do the same picture- argh, it's just not there. Sometimes is, but not always, since painting seems to be some weird alchemy of you and 'call-it-what-you-want'(= practice, skill, influence, memory, inspiration, higher power, subconciousness, consciousness etc).
The good part is, creating is great. We love it. We do it again and again. And let's put things in perspective. The 'risk' of destroying your painting by continuing it is in this world a pretty small risk.
My advice? Leave it be when you don't know if it's done or not. Give it time, and come back to it later on. Usually, the choice has been made by then.
Here, pic 40, my continued small water color. I named it 'Crazy Muppet' because it reminds me one of those Muppet characters.