When a painting isn't going right, I paint over the bits I don't like.. even if I'm almost done. It's going to extend the time you work on the painting, but you'll never get over that little or big mistake in the painting. Might as well trash it if you don't feel like going through all that work.
Working on a painting now, I've had to do that three times already... and now I'm waiting for the fourth to dry (acrylic, won't take long). It's one of the prettiest paintings I've done (I'll put a picture up later) and a new series of works, some of which I intend to sell or exhibit. You can't exhibit something, sell something, give something, if there's a mistake on it.
If you're tired of painting, and it's time the painting is done, be done with it. If it's aesthetically pleasing, go with it and send it out into the world. If not, put it aside and try again later when it's more fresh and you're ready for a big challenge.
Sometimes we need to stop, eyeball our situation, and decide whether to power through, put it on the back burner, or just stuff it in the garbage. Whether that is a painting, a novel, a friend or other relationship, a home improvement project, a blog post.. lol, career, etc - (just random ideas). If you're not willing to put your guts and heart and pulsing mind into the thing, then it's time to do some triage!
Some things are obviously harder than others.. paintings are simple.. just gesso over the canvas and start again, or hopefully your palette colours are still wet and you can just go over it.
Right now, only my paintings (I paint a few things at once to minimize paint loss from drying) are needing triage, happily.
This pic came from my favorite site: Cute Overload.