How do you leave a manipulative cheater?

I’ve been dating this girl for just a few months and she was my best friend for even longer. I adore her, but she makes me sad more than she makes me happy. Don’t get me wrong, she does make me happy, and the trust was all there until she gave me a reason to worry. She admitted to cheating at the beginning of our relationship with an ex, and I’ve seen her cheating on her phone. If I try to talk about how I feel, she shuts it down and makes me feel bad, saying things like “so you don’t trust me?” or “why are you bringing up my past?” even though it wasn’t long ago at all. She’s told me she’s scared of me cheating multiple times and I always reassure her and have given her my passwords and anything that she asks to make sure she knows I wouldn’t do that to her, and it still isn’t enough. I’m asking for advice, yet I don’t know if I’ll even use it. I don’t want this relationship to end, because I really do love her, but I have a constant pit in my stomach that won’t go away, and I know she’d never help me get rid of it.