Throwaway account for reasons.

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So my (27F) boyfriend (28 M) and I have been in an open relationship ever since we met and are getting married in a couple months. We both do not believe in monogamy and are cool with each other having fwbs and casual dates. Trust, honesty and openness has been a major part of our relationship. We tell each other everything including about our dates and the extent to which it goes. We are genuinely happy for each other. I must note that I when we met, I had severe communication and trust issues due to a lot of childhood abuse and had to work a lot to overcome them. He's been the best thing to ever happen to me and has helped me become a much better person. He feels the same way about me too.

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He has had a few fwbs in the past year and I've had just one fwb whom I share a very good connection with. I had to work a lot to improve my communication but now I've reached a point where I can be incredibly vulnerable with him and share every single detail with him. He says it's the same for him too. So, here's the issue.. Recently he went on a date(?) with one of his good friends who is in a rocky monogamous relationship that might end anytime. After the date, he told me he had a great time and they got to know each other a lot more but it didnt go anywhere physically with her. I was happy that he had a great time but confused a bit coz she's still in the monogamous relationship. Anyway its been a few days now and today while we were talking, he tells me that she was heavily flirting with him on the date and when he went in for a kiss, she rejected it. The reason he didn't tell me earlier is coz he felt extremely embarrassed from the rejection and felt bad for putting someone in an awkward situation. I told him I'm hurt and need time to process. He feels horrible for hurting me and says he just needed time to get over the awkwardness before telling me.

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So AITA for feeling like  a) He hid it from me and it's not fair ? I had to work through severe communication issues to be vulnerable with him and it feels like he shouldn't have hidden it from me. b) Trying to kiss her even though she's still in the relationship? I know she's super interested in my boyfriend but he should be more responsible? 

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AITA or am I overreacting? I really need help understanding. Any input is appreciated.

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Tldr: In an open relationship. Boyfriend went on a date and hid the details of it from me for a few days. AITA for being upset or should I stop bothering about trivial things?

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