I don’t know what to do about my relationship

My boyfriend and I have been dating for 6 years. The first year or so was pretty rocky due to him not saying I love you back for a longgggg time. In addition to some other red flags that I saw that I honestly should have just cut and run then. Anyways, eventually he said it back, but still was never very forthcoming with it, only said it as a response to me saying it mostly. Discussions about this and my needs for it more always changed nothing and the answer was that’s just how he is. Eventually I got over this and learned to accept it. Told myself I knew he loved me because xyz so I didn’t need to hear it all the time. Our relationship continued and progressed and was overall a good enjoyable relationship. We moved in together, etc. fast forward this September he brought up marriage / kids and how he wants neither but loves me wants to be with me etc. I have never really wanted kids so that was fine, and as I’ve gotten older I have come to have some negative views on marriage as an institution, so again, fine. I don’t care about marriage per say as long as I feel secure, and I want to know and feel like the person is committed, that’s all. Don’t necessarily need a legal document for that. But when I asked if he saw us together in like 5, 10 more years he said “I don’t see why not”. Which hurt. It has been 6 years already what do you mean? That’s so nonchalant. But ok whatever. Then he brought up about how he feels he may need to have a threesome one day. This is something he has mentioned before, and he knew it bothered me. But did not stop. Eventually I agreed to do some snap chatting nudes with him with a Snapchat group, to make him happy, and he did not seem to love doing that too much and it stopped after awhile. It was like the idea was better than the reality for him which I knew would be the case. I figured that would settle it but he would still sometimes bring up threesomes in dirty talk when we’d have sex. To be clear also not jus another girl but another guy too, he said it would be hot to see me getting fucked by another guy etc. this upset me too but he explained it’s not that he doesn’t love me etc he just doesn’t necessarily view sex and love the same way I do. Anyways, I would eventually tolerate this dirty talking and sometimes could get into it tbh. Just the talking about it, not actually doing it. He asked me to do all kinds of new things in bed over the years, pegging included. I would never have initiated that myself but I didn’t hate it. Anyways, September he said he may need a threesome and to explore some things eventually. This threw me and I cried so hard. Finally after a few weeks I got up the courage to basically give an ultimatum that if he needs that I can’t be a part of it and that is my boundary and line I can’t cross. He said he would never throw everything away over that and that he doesn’t need it it was just a fantasy and that he kept bringing it up cause he felt I’d never gave him a straight answer about it (even though I have gotten upset about it in the past). I asked him repeatedly if he was sure. He said yes. In addition, he has been very moody and cold on and off throughout the relationship but much worse in the last year. He always blamed it on hating his job but never got a new one. I could tell he really did hate his job so I believed him but it still took a toll on me as he would ignore me give me no affection and be very cold and withdrawn. I know he did break up with a previous girlfriend over hating his job and felt he needed to reset so he broke up with her years ago. He also would sometimes push friends away too when he would get in a mood so I told myself it wasn’t just me (although most times it was just around me and when we were with his friends or family he would put on a happy mask and as soon as it was just us again it would be nothingness). So fast forward I gave the ultimatum he said he wouldn’t throw us away. Still very moody and withdrawn. Then a few days after new years he said he feels like he has to do some things on his own like the threesomes and he realized he s bisexual and needs to explore that and that he wants to go sailing eventually). Said he loves me so much and wants to be with me but that he can’t make this feeling go away that he needs to do these things sexually. He said it’s not that I’m not enough and maybe once he figures himself out we could have a chance again. I was so desperate and sad in the moment that I said I could maybe do those things he wants with him. I truly convinced myself I could be ok with it. He agreed we could try. I know what you will say about that but I have severe anxiety and had a panic attack and felt my world was ending and in just not as strong as I should be. Ever since he has still been cold and removed and I have been second guessing what I agreed to. I don’t know if I can do this i worry it will crush my soul. I know he cannot truly love me like I love him or he wouldn’t throw away thing we built for 6 years over a threesome. I have come to feel he is selfish and I deserve better, yet I still don’t know if I have the strength to leave him for good. I am not that strong of a person I don’t think. I may be getting there idk.

TL;DR My boyfriend of six years wants an open relationship to have threesomes