Originally Posted by Neapolitan

If you forget how to play a song you know, it could be that it is not stored in your "muscle memory." If there is any secret about playing an instrument is that musicians do not conscientiously think what note to play next like you do when you first start out learning. They rely on muscle memory which comes through repetition -- a lot of practice.

Sheet music helps to a certain extant, you know the tempo and what notes to play, the form of the song but the feel of the song comes from hearing it, imo. If you learn from sheet music, (or even by ear) don't worry about the tempo, play it as slow as you like until you have it down pat. Speed can always come later.