My ex is sleeping around and its just hell at the moment?

Basically my girlfriend and I split 3 weeks ago, very sad, lots of crying, her saying she "wasn't ready" for a long term thing and she wanted to be young (she's 21). As it then turns out through mutual friends and me still being "friends" with her sisters on FB I have seen she's sleeping around a fair bit. At least 2 guys that I know of and probably more. The very idea of it is just killing me, we were so happy and she wasn't ready for a big serious relationship then fair enough, but the idea of her with these guys is shreading me apart. I can't sleep at night, I'm flitting between very angry and just 100% sad. She was always playfull in bed and yes I get she's doing nothing "wrong"... but seriously. My beautiful girl who I shared all those times with is now just out there getting as much d*** as possible... this is horrible, how the hell do I handle this?!
My ex is sleeping around and its just hell at the moment?
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