The Raindrop Prélude!
I remember all the struggles learning it, it all seems so bizarre when you can play a piece with great ease, but then remember the toil it took to get its current status.
Conquering your first "big" piece is great because after that, no matter how much you're striggling with new repertoire, you can look back at how much you struggled with another piece, compare it to how you can play it now, and be reassured that with hard work and will, you can reach your goal.
Well at least that's what I like to think, I won't let Beethoven break me!