If / when you don't feel like playing, don't play. Doing so creates an association in your mind, albeit perhaps only subconsciously, where poker, even if it isn't unpleasant, isn't fun either.
Also, to whatever extent you can, focus on the fun of playing, not the money. Yes, I know it's more fun to win and also to win more, but if your motivation for playing includes entertainment, competition, etc., then it's not just about money either.
Unless you're playing for amounts that are meaningful to your life / lifestyle, how much does the amount you're likely to win or lose in a given period really matter?