Long read, but need the insight. Ex/current g/f and close friend hooked up

I'm using this platform as this situation is embarrassing as fuck. To keep it as brief as possible here's the back story.

My current g/f have been on and off for years (she's a few years older, I wanted to establish myself in a career first etc. etc.). Well I broke up with her back in 2017 because I didn't know what I wanted and didn't want to string her along. We got back together then split up December 2018 and have been back together again since 02/2019. When we initially got back together I started noticing different things she had said previously didn't align with what she would tell me currently. I kept calling her out on her inconsistencies (I'm a therapist so it's second nature) and then she dropped the bomb on me

She told me a few weeks before we got back together in 2017 she and one of my closest friends hooked up. Someone I went to combat with who saw me at my worst when I got the infamous "dear john" letter. The way she described it is that she went over to his place as there were supposed to be other people coming over. He started telling her that I was fucking/dating other people and essentially shortly after made a move on her and basically went right in for the fuck. She said she opposed it and everything at first and basically said fuck it. She said she just felt the need to be wanted as I left her and made her feel unwanted. She said a few seconds after it started the guilt, shame and remorse hit her and she basically stopped the act and left. This is a brief version of how it was explained to me. She said she never had a thing for him as I had always felt like she was interested in him, but didn't know if it was just me being insecure.

So here's my dilemma. This was kept from me from both of them for 2 years. I remember asking her when we got back together that I always felt she had some type of interest in him and if anything ever happened; she denied. I have cut my friend out completely as he knew I still had feelings for her despite not being together. He also tried to tell me it was a mistake and they were drunk; I kicked him out of my house as I felt using the alcohol excuse was an easy cope out and felt like he intentionally set the situation up to fuck her. I don't know if she's being truthful about the situation. I don't know if I can let this go as I feel it's a boundary that should have never been crossed whether we were together or not. The relationship has been great since February, however just the initial spark that we once had, to me anyways, seems lost and i'm forcing myself to love her and remain attracted. I'm just lost and have no idea what to do. I get where she's coming from wanting validation after a break up, but one of my best friends? Then lied about it for 2 years? I feel like such an idiot. Do I let this ruin the relationship? Am I disrespecting myself staying in it? So many questions. Any insight please help. This has me fucked up