You just need to start forcing yourself to play on hard. Even though you'll suck your first play through, it will be a lot more fun. And the only reason I made the jump a week after I got GH2, was because I dominated medium and my brother didn't want to play co op unless I was on hard. So I just said whatever and got used to it. =)
Yeah. You just have to learn to move on. Keep playing Medium (or whatever), and when playing that difficulty, you will go, "I'm pretty good with Medium and getting a bit tired of it. I will try Hard (or whatever) today" and your expierence is partly refreshed.
GT: b trumpet http://club.ign.com/b/contrib?username=b-trumpet <-- Reviews made through IGN