I can't do this. Seriously.
3 weeks we've been broken up now and I know she's been going with other guys.
Honestly, apart from manning up how can I get over this? I'm absolutely torn.
I'm going to make some assumptions here so forgive me if I'm wrong, I don't know your back story, plus I'm subconsciously (not so much) giving advice to myself, but the point still stands.
She would have slept with other guys before having ever met you, and if you two hadn't have met, she would probably be sleeping with someone else right this instant. Thus really the issue here is a sort of possession mentality, and that's quite normal in a relationship. It's quite easy to assume some sort of control over each others lives once together, you 'belong' to one another, she's MY girlfriend or MY boyfriend. But really all relationships do is create a non-verbal agreement not to sleep with anyone else and be trustworthy. Once that relationship ends you have no say over what that person does, and rightly so, you are your own person as are they.
The main thing that hurts after breaking up with someone (after the initial event) in my opinion is jealousy, which would ultimately lead to resenting that person, and as once said, "Resenting someone is like drinking poison and waiting for that person to die" or something like that . You're probably worrying she's moving on first, she's going out socialising, she's happy, that she might 'belong' to someone else now. But just realise you never had her in the first place, you merely shared a part of your life with someone which has come to an end, as does everything. The only thing to do now is to start making new memories and focusing on a new part of your life, maybe sharing it with someone new.
Self-therapy man, helping myself by helping you.