Oh that happens to me too! You shouldn't let it stop you though. Sometimes I need to paint some ideas several times until they look satisfying to me. Besides, the harder it is to draw, the more you learn.
Oh bummer. Time for art seems to be short for everyone (I've already submitted to live with constant sleep deprivation). But sketching is good also and many times really relaxing and fun. And I must agree with you about the nodding emoticon.