Should I stay or go?

My girlfriend and I have been dating for 2 and a half years. Within the first few months of our relationship, she had clearly shown interest in another guy, (let’s call him Jeff) by doing things such as posting his initials on her Instagram notes. Me noticing this, i confronted her about it and we moved past it, i trusted her again because she was honest about it, apologised, said she wouldn’t do it again.

A few weeks later, everything’s going well until i see a notification from Jeff on her phone. She tried hiding it when i saw it, bluffing better than a professional poker player. I didn’t say anything about it, just played dumb. I checked her phone after and magically Jeff was gone off her phone.

Now in the present day, i get a call from her friends saying that this same exact guy, Jeff has been in my girlfriends dms 24/7 and there’s even been times where they’ve seen each other, BUT only when she’s with her other female friend (still, they’re seeing each other.) Jeff apparently calls her when he’s drunk and confesses that he has feelings for her, and she entertains but promised she never said anything like that back.

At first i was going to break up with her, but I told her that she has one last tiny window to prove i can trust her again. She promised to never do this again, removed him, said she will do whatever it takes to build my trust back… I’ve realised maybe i cant fully trust her again. I’ve been reading other reddit story’s that are similar, and i can relate too much. At the same time, i feel like if i stay around and observe for longer (part of me feels like i’ve done enough observing) then it will build my trust back. She’s almost perfect in every other aspect in the relationship, we’re both god centred, both gym together etc.

some mature answers here would help a lot