It's not healthy to forget. Being betrayed is part of your life. Right now it's recent, so it's a very big thing. As time goes on, if you acknowledge and resolve the feelings that come with being betrayed, it will become a smaller and smaller part of your life. But it will always be part of your life.
Look, your Xgf's actions say nothing about you except that she thinks she prefers someone else. But you, too, have preferences in who you like, want to spend time with, want to get into bed with. Often our evaluations are mutual - 2 people want to hold the other in the friend zone, 2 people don't want to see each other ever again, 2 people want to be lovers. But sometimes attraction is unrequited. I doubt that Sophia Loren would have picked me way back when, no matter how easily she mesmerized me. You know these things, right?
NC - No Contact - should help you let go of the betrayer, but it takes time. I suggest getting a copy of Dorothy Tannov's Love and Limerence. That may provide some guidance.
I'm sorry you've been dumped. I know it can be agonizing - but that's temporary. With a little luck, in 5 years, you'll remember this time as a blip. I know that seems like a long time, and it is at your age. But you can heal.
Personally, my way would be to feel the pain. I spend some time by myself crying, raging (hitting a punching bag, for example), feeling sorry for myself, feeling the fear of being alone, feeling the shame of not being preferred, etc., etc., etc. But when I was in school, I didn't know that would work.
IMO, a good IC can help you surface your feelings and process them out of your body. IMO, that's the quickest way to heal. But there's no way through this that happens fast. It always takes longer to heal than anyone wants it to take.