Many artists speak to me of their fears of 'overworking' their paintings. Often I find this fear prevents them from pushing their paintings further for fear of ruining them in the process. I don't believe that overworking is necessarily a by product of working too long on a piece. It's working too long without a clear concept or direction. Or, let's put it this way, overworked pieces can still be revived, sort of like what I hope a vacation will do for me.
When at a loss, put the overworked painting aside. Somewhere that you can glance at it now and then. One day it will call you, when you're not so invested in it, and you'll know what it needs and you can either fix the problem right there and then or start a fresh painting right over the previous one if need be. Paintings painted over previous paintings are some of my best paintings, some of the history of the previous will show through adding a new dimension of beauty. The new marks will be fresh, because you are fresh and your vision is clearer. I have resuscitated many a painting this way. Stopping a work prematurely from fear of overworking to me is worse, it is playing it safe, where is the real 'creation' in that? Creation involves risk and new discoveries.