Im sorry but fuck all the other people on this thread saying “don’t tell him.” Tell him everything that happened and provide PROOF. That man has no idea that the MOTHER OF HIS CHILD hooked up with your boyfriend. It’s not about getting revenge or anything, it’s about treating other people how you want to be treated by telling him. You also obviously haven’t gotten over it either. Let me tell you this and I’m sorry that it’s going to come off harsh but I was in a very similar situation as you and you need a wake up call, FAST. All the bullshit your boyfriend has fed to you, all the phrases like “she doesn’t mean anything to me and the I’ll drop her Bc she doesn’t mean anything compared to you and I’ll be loyal to you, the you’re the best girlfriend I’ve ever been had and the girl I’ve loved the most” it’s all bullshit every guy says when they get caught. Your boyfriend did not care enough for four entire months, it wasn’t even a one off hookup. He cheated for FOUR MONTHS. He clearly doesn’t love you enough to be loyal to you so he hooked up with whatever piece of meat he cheated on you with for FOUR MONTHS. And honestly don’t blame yourself for not being enough. It’s never your fault. The dude is just an asshole.
Also, another thing, once a cheater, always a cheater. He may not cheat now, he might not even cheat in a few months, maybe for a whole year if he’s rlly good lol. But years down the road? Forget about it. he’s going to 100% cheat on you. Every guy I know when they get caught cheating by their gf, they ended up acting super good for a short period of time but they always end up cheating on the SAME GF THEY HAD FOR THE SAME REASONS THEY HAD BEFORE.
I really feel for you, I really do. I was in a similar position. I got cheated on for months on end, believed all the BS they fed me. But you know what, you never get over the cheating. Do you really want to marry and respect a person that started off the relationship disrespecting you? No. Either one of two things are going to happen. The day is gonna come where you’re going to have the realization that there are going to be guys that truly love you and won’t ever cheat on you and you realize you’re not happy with the person cheated on you. It’s going to eat away at you. ORRRRR you know what’s even scarier? That you’ll keep lying to yourself that they love you and won’t ever cheat on you again and you’ll continue that relationship with a metaphorical blind fold on.
Whatever happens, I respect your decision. I wish you the ABSOLUTE best because you deserve it. We all do.
Also, don’t forget that if you never caught him cheating for those four months, he would’ve kept it going for who knows how long before you found out/ never found out.
Valid reasons for telling someone something (whether that something is an apology, or an admission, or something like this) is because you honestly and legitimately believe that doing so will in some way make things better for them. That’s not to say that if you are mistaken, and they end up not being better off for having heard, that you have done something wrong, only that your intention, and your honest belief, be that you are helping.
If telling someone something will serve no purpose other than to make you feel better at the expense of making them feel worse, then you should keep your mouth shut. As /u/Finn_Finite says, closure comes from within; it’s no one else’s job to provide it for you.
So if your motivation for wanting to tell him is because you legitimately and honestly believe that it will in some way make his life better to know, then tell him.
If your motivation is either for your own sake, or simply to punish her, but doing so is likely to make his life worse, then hold your peace.
He definitely has the right to know what happened. Just be aware that it will most likely destroy your relationship as well. That being said, is your relationship really worth saving?
Dump him and then send the proof to the dude.
Think about whether or not you’d want to know in his shoes. I, a 27 y/o guy absolutely would. I almost feel that withholding the information is forcing him into a relationship he doesn’t know that he may not actually want, with a person he doesn’t really know. So personally, yes, I would absolutely be telling him. Not necessarily for revenge, although i’m sure that would be a factor, but for the guy to have the ability to make informed decisions about his relationship.