27F, 38M—should I end my 7-year relationship because my fiancé is considering being with two women without me?

Here’s a rewritten version of your text: --- This is quite a lengthy tale, so please bear with me. I’m a 27-year-old woman engaged to my 38-year-old fiancé. We’ve been together for five years and engaged for two. To provide some context, we’ve been in a monogamous relationship since 2017. I’m a bit of a nerd and tend to be the odd one out in my family. My fiancé is incredibly supportive of my interests, and he comes from a family where love isn’t always openly expressed. He stood by my side when my mother lost her battle with cancer and even received her blessing. Likewise, I supported him when his stepfather passed away. He also helped me realize that some friends I trusted were actually toxic influences in my life. Thanks to him, I've been able to make new, positive friendships, for which I’m very grateful. He has truly been my light for so long that about two years ago, I suggested we explore each other’s fantasies. He was on board with the idea. For me, it was him dressing up as my favorite male character from an anime and attending a convention. For him, it was a more intimate fantasy involving a threesome—he wanted to be with two women. I told myself I would go along with it if he kept his part of the agreement. When the convention rolled around, he didn't accurately portray the character I had in mind. Instead of wearing the messy red wig I had for him, he spray-painted his buzz-cut hair and acted cold and rude throughout the event. Later, I learned he had been distracted because there was a popular voice actor at the convention whose autograph he wanted, but he felt self-conscious about his look. I was disappointed but reminded myself that we had an agreement. When he brought up the idea of a threesome, I assumed he meant hiring an escort or someone familiar with such arrangements. To my surprise, he suggested a mutual acquaintance named Serena. She had a tumultuous background and had known my fiancé for over a decade. Although I was uncomfortable with him wanting someone he knew, I decided to let it go, understanding his perspective. On the day of our planned threesome, I panicked and backed out. Earlier that month, he had mentioned I had gained weight, which triggered my insecurities and made me feel unattractive. I worried he might leave me for Serena, compounded by my family's past messages that there was something wrong with my interests in video games and anime. I’ve struggled with low self-esteem and emotional neglect, which has often led me to doubt my worthiness of love. Thankfully, I’m in therapy and working on overcoming my people-pleasing tendencies. When I called off the threesome, he was understandably upset, and we agreed to move on without discussing it further. However, the topic resurfaced five years later. We work in healthcare and finally had a day off together. After a lovely day, we began to get intimate, but he suddenly confessed he couldn't stop thinking about having an experience with two women and contemplated doing it behind my back. That broke my heart and completely killed the mood. He reassured me that it was all right because he never acted on those thoughts, but I still felt devastated. A couple of days later, I opened up to my therapist about it. She suggested a separation but was understanding if I wasn’t ready for that. I experience anxiety, and my understanding of relationships is primarily shaped by what I've heard from older family members, which often involves stories of infidelity. This made me reflect on our relationship—he waited over five years to propose, and when he finally did, it wasn’t a traditional kneeling moment. On Valentine's Day, he often gives me discounted chocolates, and he rarely chooses thoughtful gifts for my birthday. During our date, he disclosed a personal secret of mine to his friends, which embarrassed me. When I told him it was inappropriate, he dismissed my feelings, insisting that I shouldn’t be ashamed. Since the argument, it’s been a week, and I’ve been in a state of depression. I’ve taken off my engagement ring, and I think he’s noticed. He keeps apologizing, acknowledging his mistake, but I’m not convinced he understands what love truly means. No matter how many times I express my feelings or concerns, he seems to shut me down. I worry that I might be overthinking things. I used to turn to my mom for support, but now I rely on my therapist, who is far away and not always available. If this relationship is indeed toxic and I choose to separate, I’m uncertain if I’ll find someone who can accept me. I welcome any advice and appreciate you listening.