Help

So me(17M) and my gf(17F) broke up around 4 months ago. We had been together for two and a half years and one day she randomly stopped talking to me at school and stopped answering my calls or messages. I was very hurt but decided to not show it because i was waiting for her to come back to me. That never really happened and here i am. I feel like she moved on too fast from us because two weeks after the "break up" i found out she was texting another guy. I need help learning to move on from this. I loved her a lot and I still do. It just seems so hard for me to move on meanwhile there she is texting another boy two weeks later. I am struggling with mental health and self-worth a lot these months, i have recently started going to the gym and that has made me feel better. Any type of advice from someone with a bit more experience would help a lot. :)

(sorry if there are any grammatical errors, english isn't my first language)