An interesting phenomenon: the very first time you play a song, that you know by heart, is much easier than the second time. My explanation for this is, that the first time you are inclined to listen to the well known song in your head with great attention, but the second time you try to retrieve what you just found that first time. What notes did I play again? So you don’t listen so well to the song in your head anymore. Now you are much busier in your head, thinking a lot, and that hinders playing by heart.
TIP:
So don’t bother remembering the notes you found. You will find them again next time and then probably a bit easier. And remind: you will never forget the song itself!