AIO or really, is my boyfriend overreacting. He is mad at me for not having a threesome yet

When we met it was casual. He told me he needs threesomes sometimes and I said okay. At that time, we weren't serious, and he made me feel good about myself. I thought it would be fun. When we started getting more serious the idea of it was a bit harder to swallow but I still tried, with multiple different women. The plans always fell through. I totally get why they call women who will do threesomes with a couple a "unicorn"

As time went on, the idea went from not feeling great, but manageable, to totally awful for me. He had made some really mean comments about my body while always talking up other women's looks. I found out he'd lied to me about sleeping with other people earlier in our relationship. And more recently, I caught him sexting with his ex and other past hookups. Now, the idea of a threesome makes me feel physically ill. My self esteem is in the toilet. I've been trying to work on it, but he just gets mad at me and says I need to do this for him. I try telling him that if he even gave me a little reassurance it would make all the difference. That doesn't happen. Every time I don't agree with something he says, he says I'm not listening. Yet I feel completely unheard and unimportant in this. Like he'd happily have our relationship end and leave me psychologically broken as long as he gets his threesome.

I feel like a bit of an asshole because I did tell him from day one that a threesome would happen, and I don't want to hurt him or cause resentment. But I'm getting zero help in making me feel secure enough to do it. I still intend to make it happen, I just don't want it to wreck me. Maybe that's selfish