How do I [28M] evaluate whether trust can be rebuilt after a complicated start with my girlfriend [20F]?

[MKF]

My girlfriend [20F] and I [28M] have been dating for three months. We explicitly agreed to exclusivity last month.

Then we were away for 2 weeks. After she returned to her city (i was still away) and, obviously, after the exclusivity conversation we had, she arranged to meet a man she had been emotionally and sexually involved with before me and hadn’t closed things with. From what she wrote at the time, it appears she intended this to be their final meeting and was already trying to end their unhealthy attachment. She had even prepared a goodbye letter.

Nevertheless, they went drinking, returned to his apartment, and had sex. She went back the following day as she had forgotten her ring, and they had sex again.

The arguments in her favor are:

* She was leaving a complicated and possibly manipulative attachment, rather than beginning a new affair.
* Before I confronted her, she had already decided internally that she needed to end things with him.
* The meeting seems to have been intended as closure, even though she failed to follow through appropriately.
* Since the confrontation, she has ended contact, accepted monogamy, apologized, and appears genuinely committed to changing.
* She says she cannot repair the past but wants to make things right in the future and does not want me to stay out of pity.
* ⁠She was constantly manipulated by that man she met, and she had trust issues towards men because of what she experienced in the past

The arguments against her are:

* We had explicitly agreed to exclusivity before she met him.
* Whatever her intention for the meeting, she still chose to go home with him and have sex.
* Returning the following day makes it harder to view the first encounter as one final mistake.
* She initially concealed or denied important facts.
* She herself considers it cheating and has said that, in my position, she probably would have left.
* Cheating has always been a firm boundary for me, and I worry that resentment and intrusive thoughts will prevent me from genuinely rebuilding trust.

I love her and believe the situation was more complicated than ordinary cheating, but context does not erase the agreement she broke. I also do not want to stay only because I am attached to her, nor leave automatically if genuine repair is realistically possible.

Given both the betrayal and the surrounding context, how should I decide whether to stay and attempt to rebuild trust or leave the relationship?