Long story...

I've been with my partner since we were 15 years old, going into our 24th year. We had kids young and stuck it out when we probably shouldn't have but have the best relationship now. Probably 2 years ago I started to realize that I've missed out on dating, basically all the stuff you do when your young. I've never been with anyone else and still don't have that desire but the thought of him doing that excited me. It took a solid year of talking about it before it happened. For the first 5-6 months he was solo and there was good parts and really bad parts. I felt very left out. So we decided on maybe having a situation where himself, his fwb and myself get together and he does stuff with both of us and we just all have a great time. The 1st time was definitely one of the best nights of my.life. it was better than I expected and I believe for the others as well. We had rules in place and it worked out. For the next time we had added a few things in that we had all agreed upon. Well this time in some ways it was great and others it wasn't. They thought it was great and loved it since they were together at least 80% of the time. I was very much left out. I tried to insert myself but it didn't work. Now I'm not sure what to do. I've shared this and nothing more has been said. Do I bring it up again? Leave it alone and countine? Or end the whole thing? I really do like it but I don't want to build resentment in my relationships. Thanks for reading and hopefully someone has some good advice for me!