TIFU as I presented my bf with his cheating partner on a silver platter

I (f30) have a boyfriend of 3 years (we will call him Sam(m33) and in the beginning he lost his friend's group because of a fight so he was pretty alone but that wasn't really a problem as we were happy to do everything together.

Anyway he was in the military years ago and sometimes talked about his time there and his buddy which we will call Leo (m30). I sometimes made jokes about their gayness for each other as they seemed to be really close.

A few months back I wanted to surprise him and find Leos contact as I thought it would be nice if they could talk again as they lost contact over the last 10 years. I found him on Facebook and gave him Sam's email and he was excited and thanked me and told me he would write him. Sam was super happy to hear from him and was excited to meet up with him. He asked me if I wanted to come with him but I told him I could meet his buddy another time, they should use the time to talk and get to know each other again after all those years.

Everything seemed cool and they sometimes went for beers or watched hockey games and they got really close again. I was so happy because he seemed to be less depressed and actually went out again. I actually met Leo once when we all had breakfast together and he was a really nice guy.

A months ago I noticed that they met up more often and as happy as I was I was also a bit bummed because he had less time for me but it felt selfish to tell him that so I was happy for him. Then in retrospect I noticed that we had less Sex. Sometimes before we had Sex like 3 times a day and sometimes we didn't do anything sexual for almost two weeks. With two full-time jobs and my health problems it was always a weird "schedule" but I noticed it got less and less. Usually he would initiate it more often then I did but then I had the feeling it was always me who asked or tried to feel him up. Again I didn't talk to him like I probably should have and thought okay he is working more hours he is probably tired and also depression is always a bitch to deal with.

Fast forward to yesterday when I wanted to take a picture of our cat who was sleeping in a super weird and funny way I couldn't find my phone so I used his (we were always super open about our phones and have the same passcode) and I saw a text pop up with kiss emotes and curiosity got the best of me. I opened it and it was Leo who told Sam that he missed him and was excited about the weekend where they would meed again and told him he had a naughty surprise. I was shook and scrolled further up and my fear got confirmed as they apparently have been sleeping with each other behind my back for at least a month if not more. He came back downstairs so I put his phone back where it was and didn't say anything. I was so shocked. I went to bed early and said I had a migraine and he knows I then need darkness and no noise so he went to the living room to watch a movie with headphones. I cried and cried and cried and felt so much anger? I brought this person back into his life, I basically presented him on a silver platter! But on the other hand if it weren't Leo who knows if Sam was sleeping with someone else. Did he do it before? Was our entire relationship a lie? I never got the impression that he wasn't happy with us. Our Sex was amazing. I wanted to marry this man and have his babies. I feel so sick to my stomach. I haven't talked much with him today but I can still pass it up as my migraine. How do I even start a conversation about this? How can I compete with a man? Someone he has known for so long. Did they already sleep with each other when they were in the army? I am so confused and hurt and angry and I just can't comprehend all this.

I just need to get this out. Tomorrow I will think about what I will do.

Also I am sorry English isn't my first language and I am sorry if this is all over the place and has mistakes I am still bawling my eyes out.

TL;DR: I found an old friend of my bf on Facebook and urged them to meet again as my bf had no friends after a fight with his last friends circle and he cheated on me with his army buddy.