Hey OtherAngel! Up top I wanted to say that I saw that you attempted to post this thread twice - I approved this one but in the interest of keeping our boards tidy I did not approve the other one. They seemed really similar, so I hope that's okay! If there was information in that one that did not make it over into this one, feel free to add more.
I'm sorry you're feeling so bad right now - I think I know exactly how you're feeling, too. I feel comfortable sharing with you that I was once in a very similar situation a few years ago: I discovered my boyfriend of several months was secretly in another relationship the entire time we were dating. I don't think you're blowing this out of proportion at all. Even if it was a one-time weekend fling, it feels awful to have your trust broken and know you didn't have all the information, especially because it sounds like you would have chosen something different if you knew it. As for whether or not you should or should not feel mad or used - you're allowed to feel however you want to feel about this situation, or any other situation for that matter. There could be folks out there that would feel differently if this happened to them, but what matters the most is how you
One thing I will say along those lines though is that I don't think you need to be mad at yourself for not asking if he had a girlfriend. I believe the assumption you made about him being single is very reasonable, and I think expecting him to be forthcoming with information about his relationship status is also. Something I wanted to throw out there as a possibility is that some couples have consensual non-monogamous relationships - this means that they have decided to not date or have sex exclusively with each other. If you're interested in learning more about these types of relationships, you can check out this guide
. There may be an arrangement between him and his girlfriend that you are unaware of. If that's the case and they are non-monogamous, I do wish he thought to be more up front about it with you.
It's my personal opinion that you should not
message his girlfriend, even if you think she should know the truth. I say this because this issue is between you and him, not you and her. What do you think about reaching out and telling him how you feel instead?