How do I get over wanting to beat my girlfriends rapist?

My girlfriend got raped a few months ago we were together. She was hanging out with her friend who she thought was her best friend. They went out for lunch and after he stopped in a park and asked for oral sex.
She tells me she said no and she had a boyfriend and didn't see him like that.

She tells me this guy lost his shit and started demanding saying I was not good for her he was better blah blah she kept refusing and he reached out and hit her and forced her into oral sex using physical violence.

After some time of me being pushed (I told her this fat piece of shit was bad news since day one) and me thinking she cheated we finally found evidence and tried pursuing charges but the police didn't do shit.

Now I'm super furious I know this guy and he has been mocking me and saying I'm a cuckold claiming my girlfriend cheated on me and while her friends dont believe she wouldn't and that she has never been into him it still pisses me off and I want to rip his head off and I can't get over it it eats away at me knowing someone else had sex with my girlfriend while with me.

I know it's not her fault but it bugs me I'm going to therapy but it isn't working. I've even considered a break up tbh. I dont know what to do
How do I get over wanting to beat my girlfriends rapist?
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