My boyfriend is furious that I’m breaking up after he insisted on opening our relationship

My boyfriend and I (both in early 30s) have been in a relationship for almost four months. From the beginning, the topic of an open relationship came up, and I was very clear that I wanted something exclusive. Eventually, we agreed that we could revisit the discussion after one year and see how I felt about it. At the time, I already suspected I wouldn’t be okay with it, but I liked the idea of spending a year with him, so I thought I could cross that bridge when we got there or we can break up with good memories.

However, during a chill night together recently, he suddenly brought up the topic again and told me that he definitely wants an open relationship after one year. It was kind of out of nowhere because it was not even one of our topics recently. This instantly made me sad—not angry—because I was already struggling with the relationship in other ways. I had been trying to convince myself that I could deal with certain issues, such as his tendency to be selfish in many topics, ignoring my emotions if they are not matching with his, and a sex life that hasn’t been as fulfilling as I’d hoped (even though I brought up the subject so many times).

I told him that his timing really upset me because I wasn’t prepared to have this conversation, especially when we were already dealing with other issues. He apologized for bringing it up after seeing how sad I was. But the next day, I realized I was done.

When I told him, he got angry and upset, saying I hadn’t even thought it through, that our sex life was just fine, and that I was making up problems in my head. He also said that we had promised each other a great one-year relationship, and now I was just giving up.

I’m actually not someone who quits things easily. But the way he handled this topic—with such a strong focus on what he wants, rather than how I might feel—made me even more frustrated. And at this point, I don’t even feel like I can bring up how I feel, because it would just make him angrier and he wouldn’t consider what I say.

Now I’m wondering if I should have even started this relationship in the first place.

Am I being unreasonable for ending things? I do not want to feel guilty..