You're 17 dude. There are plenty of girls out there, keep you chin up. It's just one girl. She may have been important once, but now that's gone. You've got to move past it and live your life. There was life before her, and there is life after her. Good luck!
Well, it sounds to me like this girl really isn't worth it. If I was you I wouldn't of even taken her back after she left her ex for you, I mean come on... Earlier she left you for him! ...Just my opinions. Someone will come for you who is true, but don't be so easy to trust and love everyone who is interested in you.
There's always going to be the girls and guys who only want to screw you over. But there ARE the ones who won't. Don't give up yet, you usually find the right one when you least expect it. Just have hope. And if you ever need someone to talk to, feel free to message me. Just an open offer.