For me the Hardest part is trying to play songs I never heard before.. I have terrible time with keeping rhythm and tempo, always did.
IF I cant hear the song in my head I cant seem to get rhythms right . Takes me ages it seems to get a new song playable. I do use now the music notation programs on my computer to hear the music slowed down and at speed but still end up needing to make mp3s of it and listen to them often to learn. IF it has words to go with it it makes it easier as I will learn to sing it to get the tempo