Practice, practice, practice. Put the tunes in your head by memory like hearing them when you are driving or walking. Repetitious playing eventually becomes second nature. Try playing your most favorite tune with out the sheet music until you get it. Then on to the next. Once you have them set to memory, you'll likely never forget them.
It might be something to do with playing by ear Vs sight reading music. Since I never learned how to read music I may have to rely on memorizing the sounds. That does not come with out the downsides, such as when I do a song, some one might want to argue the way it is supposed to sound which is a real problem if you are playing music in a band and the miusic should be played the same. But then to I do a lot of solo music or folks join in to my beat of the march.
Mudjack