Breakups

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So, me and my ex broke up a couple weeks ago.
We were going out for a bit over a year, really beautiful, healthy relationship. However, she's usually been very impulsive with her decisions and emotions, leading to a couple arguments. I am also a fairly insecure person, so I would have trouble when guys would hit on her at parties and all that (she's a total 9).

Anyway, I went to work at a camp and she went to her hometown for a couple weeks. She was partying every night and getting to know this guy really well, so I told her I was jealous. She got angry at me and stopped contacting me until we met in person. When we did, she told me she had cheated on me with this guy and wanted to leave me to be with him.
She never expressed that she was unhappy with out relationship, I thought it was stronger than it had ever been at that point. But yeah, its final, she's with him now and only wants me as a friend. I've been having a hard time with that, and I'd just like some advice.

I'll also add that she's fairly young (16, 15 when I met her). I'm 19, second year of college.