Getting over your first love?

Hey everyone, me and my ex girlfriend were together for just over a year, we were so alike, our time spent together was fun, we seemed to be made for each other, everything was good.

But recently (few months back) her ex boyfriend came into the picture, her reason for this was, he joined her group of friends at school and if she didn't talk to him her friends would neglect her and kick her from the group.

I understood and said fine, talk to him if you need to but then she started adding him on social media, she maintained the opinion that she didn't like him and she was just friends with him to stay in the group.

I told her my issue and gave her the choice, either delete him on social media and just stay "friends" in school or I will be forced to leave as I am unhappy.

I gave her this ultimatum yesterday and she hasn't text me or anything yet so obviously she has chose to keep him over me.

Me being her first love, her "soul mate", the person who helped her with her depression, panic attacks and social anxiety.

I guess I just feel stupid, she chose a guy she was with for a month, who she claims she didn't love over me who would give her everything

How do I overcome the feeling of missing her, and the relationship in general?

Thank you