Hello everyone

I'll try to keep my story as short as possible and to the point but without overlooking some core details (at least to my opinion), I hope this will not bore you and I appreciate any kinds of thoughts on the matter.

The story goes like this: about 8 months ago a friend of mine introduced me to a good friend of his that was visiting town. He said that she is great and that I should try and "make a move" on her for fun or for something more serious if I would like that. My first response was: "Sounds cool, is she easy?" , to that he replied that "no but if she likes you who knows".
I met his friend and at first nothing really happened, I thought she was very nice and intelligent but I wasn't really interested and not even that attracted to her.
A bit after I started noticing that something was there and shortly after we "hit it off". Since then, I went through the process of falling head over heals in love with this girl (as she did with me) and we've been together and madly in love ever since. This girl is truly remarkable and I really love her from the bottom of my heart (we are already discussing marriage).

In any case, shortly after we fell for each other (about 7 months ago) she one day told me that she has something hard to tell me: she had told me that in the past ( around 5 years before me and her met) she had sex a few times with my friend that introduced us. She said that she was a different person, young and in college. She said that she really doesn't remember any of it and that she had pretty much repressed it. I guess it came to me as a bit of shock, more so because her and this friend were (and still are) truly good buddies and without a hint of sexual tension.

I first of all thanked her for voluntarily telling me this and for her being so honest (she insisted that she didn't want me to hear this by chance from someone else). At first I was a bit upset, not because I thought she was a virgin or anything, we both have our histories and that's fine, but I guess my ego was a bit hurt and I told her (and still believe) that had I known before we met I probably wouldn't have fallen for her (so maybe its good that I didn't because she is the best thing that happened to me). Anyways, at first it wasn't too bad but I've noticed that as the time went by it got worse and worse, like I keep putting images of her and him in my head and it drives me nuts. Moreover, he's my friend and a good guy and i still hang out with him but its a bit of different energy, it gets worse when the 3 of us are together (I will never ask her to not see him because they are good friends and more than that he introduced us so I guess in a way I owe him).

Why does this bother me so much? I've been obsessing over this and I keep running these images in my head, I think I'm also a bit disturbed by the fact that my friend is kind of a player and treats women like ****. My girlfriend told me that after it happened back then she was kind of kicking herself for letting him treat her like ****, then she also added: "if it wouldn't have happened me and him wouldn't have been friends and I would have never met you" - a thing that is probably true .
I think over a course of a year they had sex maybe 4 or 5 times and I know they were both really drunk so I can't imagine it was too hot or kinky, she also told me that but I'm not sure if she's just trying to make me feel better.

I am not jealous of her in the sense that I think she has any attraction to him or any other guy for that matter, I trust her completely and I know she loves me like crazy...... I guess they were just buddies who got drunk a few times and ended up in bed, regardless, it is driving me bonkers.

Does anyone have any advice? don't tell me to dump her because I really don't think that something like this should be a reason for me to dump the love of my life. Am I just over reacting or am I just being insecure about something else? I have spoken with my therapist over this and I have realized that its stupid of me to obsess this way, still, I can't get these images out of my head.
Help me please.
thanks y'all