My best friend stole the lad I liked

So I was talking to this guy and I really really liked him. We hooked up a few times and he wasn’t looking for anything serious as he was only out of a relationship.

Him and my best mate ended up on a night out together and he had been off with me ever since so I know something had happened. I found out today a week later, a few days after he ended it with me, that he likes her now and she likes him and they’re gonna crack on.

I don’t know what to do because she knew how much I liked him and how happy I was. She has completely tore me down to nothing and I’ve never felt so insecure and alone in my life.