tell her you didn't cheat, won't be abused and shit on for something you didn't do, and that she should just go,then move on and ignore the fuck out of her when she comes crawling back.
Either she has endeniable proof and you got caught, you're being gaslit and buggered as some sort of power trip so she's got something to hang over your head and is willing to let the relationship hang in the balance to force you into subservience so she'll stick around, or she's the one fucking around.
If she's treating you like shit and abusing you, she's already made up her mind about you and rumors and whispers mean more to her than your own words and actions.