I get that kind of feeling regularly but I find if I just keep trying new things and honing the skills I am weak at I can overcome it and gain satisfaction again. If you are unhappy with what you previously felt to be good it means that your perception has changed and is a sign of improvement. Even though you don't feel happy you are in the stage that allows for the next level of growth as a musician and if you stick to the grindstone you WILL be able to improve. I think the idea that art is just self-expression and exploring your emotions is completely wrong, art is the field that requires constant dissatisfaction and humiliation to achieve any kind of greatness.