Most Helpful Guy
For 15 you seem pretty wise, knowing that you can't recreate what you had with this guy back then. I don't know exactly how go about forgetting who he used to be, and I don't think that is really how you have to approach it.
You can remember the good times, remember who he used to be and what you used to have, but put it in context... Obviously you're both at an age where you're growing and changing a lot, figuring out who you are and what your likes/dislikes are. It's natural that people change and evolve... That's what happens when you're 15. You had something good, and there's plenty more good out there to be had. Be grateful for the good times you had with him, and focus on the fact that you can get that feeling again, because there's someone out there who IS the right guy for you NOW rather than in the past.
I dunno if that helps, but good luck!
Most Helpful Girl
Remind yourself of the present, and who he is now. Try to let go of the idea of who you used to love, because that isn't who he is now.