Did I ruin my relationship?

I (29m) have a high school sweetheart relationship with my (30f) fiancé. We met freshman year of high school dated sophomore year broke up got back together junior year. She got pregnant beginning of senior few months later after drunken night I cheated on her With her gay best friend. I was coming out of the closet as bisexual and had some questions and we got drunk and pills were involved and it was a mistake. Long story short she knows and we moved past that, but I’ve been working on my relationship with my fiancé trying to make it up to her.

Now almost 12 years later. We are still together. We have two children and I proposed about 3 years ago, before I propose, I never wanted a marriage. I didn’t believe in it don’t believe we need paperwork to prove our love and so forth. But before I proposed, I did a little self thinking on do I want her in this forever? Truth is I do want her forever I don’t plan on living this life with anyone else. But I am also open to the idea of having an open relationship. Before I proposed, I wrote a letter stating this idea, but I couldn’t bring myself to showing it to her, so I ripped it up and threw it away because I’d rather live my life with her than throwing it all away just based off an idea. a few months later I propose because she is the only thing I truly want in life.

Recently I felt like I needed to be honest about my thoughts and fantasies. So as the woman, I love, I would hope I can share these thoughts and fantasies with her so I did. I told her about the letter and all. She assumed this is something I currently want to pursue. Practically giving me an ultimatum of you need to do it or I don’t think I can live knowing that you want to, but didn’t do it. She offered me a hall pass to get it out of my system as a one time thing. And I told her I’m not going to pursue it if it’s not something she’s comfortable with.

Weeks turned to months of back-and-forth talking about this to the point of our relationship coming to those make up break conversations multiple times. I feel like I ruined her and our relationship. She is depressed, but constantly wants reassurance. We are trying to make it work, but we are both becoming distant. I feel like she will never look at me the same and she feels like I don’t wanna be with her.

Does anyone wanna chime in and give me some advice?