Hey, I had a similar situation. My former SO slept with someone when we were first going out after telling me that she didn't want to get physical with anyone and wanted to take it slow with me. It was a total gross out presentation too as my buddy went into gory details on how his friend and her hooked up. I no contacted her and never ever told her I knew what was going on - though she contacted me months later and we ended up going out. Ultimately, the relationship lasted about 2 years.
During the relationship, I told myself I wouldn't hold this against her but every time a major decision came around (moving in, going on big trips together) all the self-doubt, the inadequacy, the fear and feelings of betrayal bubbled up. So I made excuses not to do stuff or to do a scaled back version of them, not wanting to commit to someone I knew had a flakey past. I let this eat away at me, to the point that I was always trying to keep an open lane to exit the relationship if she flaked again at the drop of a hat.
The blowback on this is that, I sometimes saw issues and demons where there were none. A pic of her and her childhood friend (a guy) made me wonder if she was sleeping with him - totally out of line (though I never said it out loud) and it isn't any way to live, worrying about stuff like that all the time. I responded by over-correcting but then I told myself I wasn't seeing things when the signs were there that she was emotionally cheating, getting physical with others, etc.
What I wish I did was lay all my cards on the table, just gush and come clean how I felt, why I felt that way, etc.. and await her response. Would I have come clean, I feel like it would have either ended right then and there or it would have continued and given me that catharsis that I needed to fully invest in our relationship.
Though we're broken up now, and though I think it is for the best for me (honestly), I do regret holding back. I think that holding back these feelings or gushing, getting resolution but still feeling these feelings years later could be a sign that you'll never get over them. For me, I wouldn't want to live with somebody I didn't trust (again).
Hope this personal story helps you make a decision.