How to get over my ex girlfriend?

So i'm a sophomore in high school and my girlfriend is a freshman. Our relationship was fine. I mean there were times when we fought but we got over it. She wrote me love letters and told me she'd never cheat on me and say she'd never leave me and would always fight for us I even went over to her house and tried to make a good impression. until recently...

My girlfriend is friends with her ex boyfriend, which he likes her and wants her back, she also lost her virginity too him which i dont really care about, but they're really close which i didn't like. Everyday I'd see him always touching her and stuff like that which made me upset and yesterday I wasn't at school and my friend texted me saying she saw him holding his hand but she said "he was warming my hands for me".

So last night me and my girlfriend broke up over the phone while she was at our friends house. and she kept calling me an a**hole and stuff like that and i didn't really say anything and she said the reason why she wants to end our relationship is because I don't like her ex and her being close so I pretty much told her to f**k off.

I'm really upset and I don't know what to do?! I still love her and I cant stop thinking about her!!I mean I erased her number out of anger and all of her pictures on my phone. My friend said to leave here alone, but I want to say im sorry and everything!! Please I need help!