Metroid Prime springs to mind, I was actually 12 at the time. I got stuck at the final boss, tried it dozens of times and kept failing. I ragequitted and didn't touch the game again untill 2 years later Metroid Prime 2 released. I bought that one, but before I fired it up I just HAD to finish the first Metroid Prime. I then killed the final boss in 1 go, and was shocked. I still wouldn't call that fight easy, since it wasn't, but I didn't even have to restart it once. Now, 5 years later, I still love both games to death and fire them up occasionally.