One of the things I love about music is that I don't believe I have ever finished learning a song, or instrumental piece of music.Starting from being completely musically challenged (about 5 years ago) to today where I think I'm almost at the point of getting it, learning music has been a continuous spiral upward. New vistas continue to open up for me as I learn more, and I continue to re-learn songs in more challenging ways. Richard Hefner has a neat banjo page "EZbanjo" that has tabs for each song in 4 levels: basic melody, basic frailing, hammer-ons and pull-offs, and double thumbing. Each successive version adds a little more complications with the new technique. Each new level that I tackle has been the hardest I've tried.
I learned a simple song like Little Liza Jane years ago, and I'm still learning it.
The hardest time I have had with music (not a specific song) is learning to go from background playing to melody playing smoothly when singing. I still mess it up about 50% of the time.
Maybe a teacher would help.
JAB