I’m trapped in a loop with my girlfriend. I want to break up every time I think about it, but I can’t bring myself to do it.

I need to vent and get some outside perspective because my head is completely fried. I’m stuck in a brutal cycle with my girlfriend and I don’t know how to break out of it.
To give some context on what’s been happening: the trust in our relationship is heavily fractured. There’s a guy she knows where she has constantly made excuses for him, hidden things from me, and turned my genuine pain and boundaries into technical arguments. Every time I try to bring up how much this hurts me and how it ruins my trust, it gets dragged into exhausting, hours-long debates.
Deep down, when I’m on my own thinking clearly, my gut tells me exactly what needs to happen: I need to break up. I’ve realized I can’t marry someone I don’t trust, and I can't build a future with a girl where I constantly feel the need to look over my shoulder or investigate the truth.
The problem is, the second I face her or think about actually losing her, I completely freeze. Every time we’ve had periods where we weren't dating, I felt a massive wave of depression, like my life was totally over and there was no one to turn to. To avoid that void and panic, I keep staying. I’m trading the sharp pain of a breakup for the slow, daily ache of carrying zero trust and constant anxiety.
Just recently, we had another massive emotional night over text about the timeline of everything. At the end of it, she just texted saying "we're okay and I'm happy you mentioned it again." Immediately, my brain felt a hit of relief because the threat of a fight was gone, but then I instantly started over-analyzing her exact words ("what does she mean by again?"), showing how high-alert and paranoid I've become over every single text message.
Right now, I've told myself I’m not breaking up with her yet. Instead, I’ve drawn a strict line in the sand: if anything happens again—even if it's small, or if I find out she met up with a guy, texted someone and hid it, or lied about a past hookup/kiss—it is officially over.
But honestly, I’ve reached a point where a part of me secretly wants her to cheat or do something terrible on purpose. I feel like if we end on horrible terms, it would be an escape hatch. It would take the guilt away, make the decision for me, and let me just be angry instead of sad and conflicted.
I know it’s crazy to wish for a bad ending just because I’m too weak to pull the trigger myself. How do I find the strength to walk away when the fear of being lonely is paralyzing me? Has anyone else been in this loop?