Whenever I fail to make a successful painting, it is because I wanted to make something beautiful. My expectations with the painting's outcome sabotage the piece in the same way that when a person betrays my expectations, the only outcome is disappointment and frustration.
When painting figures, I always have to mentally prepare myself before starting. I have to treat the figures like real people and take them at face value. I have to emotionally detach myself from the subject, and impartially take in visual information (planes, value and line). Figures are beautiful just for being figures, not because they are imbued with an artist's emotions. Just like any interpersonal relationship, I avoid creating a mental, finished and idealized version of the piece.
That's why, since I started painting, I found that it's no use trying to make a perfect piece. I make so many mistakes that in the end, the image becomes a sum of it's faults, and cannot live without them.