We have been together for 6 years and we have always been non-monogamous. Is it normal for it to still feel like such a struggle?

We were both visiting friends in town for a festival and planned to party Friday until early morning and then spend the rest of the weekend leisurely immersing in the festival. I (33F) left the party alone at 8am to have some sleep. He (45M) stayed, made out with a friend of ours who he has pursued in the past (and I was okay with it happening), he got invited back to hers with some friends and he had sex with her, no heads up or any indication this was happening while I was there, all happened only after I left.

I woke up at 1pm and messaged him to ask if he is still coming or whether he is somewhere so I can join. No reply, until hours later, when he messaged me to tell me he messed up. We met for dinner to a place he had invited me to go with him (no reservations, so we didn’t get in) and have time together, all sleepless until then. I got frustrated and angry at him for disappearing, then he told me he had sex, and I got even angrier.

I have not been the best at handling jealousy in the past, but I feel like this hurt me beyond what jealousy could do. He seemed to not even care enough to say be there early enough to secure a table, or think to invite me to where he was. We got sad with each other, later our friends joined us, at some point he disappeared outside to smoke with a couple of them. He was there for half hour so the rest of us go out as well, and he was having deep chats with them arranging a threesome with them for when I’m away for a three-week work trip.

We’ve been doing this for 6 years and I don’t think this is justified behaviour. I have not always been good either, I have issues with clear communication of my boundaries and get very emotional, but I think I should have developed some thick skin by now… thank you for listening.