I?m (29M) falling out of love with my (28F) fianc? after an 8 year relationship...

We?ve been together for almost 8 years. We met in high school, dated through college. Moved across the country together where we?ve lived together for the past 4 years. We are best friends.
Let me preface this by saying that I?ve been in a few relationships prior to her but I?m her first boyfriend / anything. We also both gained significant weight, however in the past 2 years, we?ve both lost significant weight and we almost look like different people.
3 months back, my fianc? brought up the fact that she was having ?cold feet? about the idea of marriage and being together with only one person, along with the fact that she never slept with anyone else. She suggested an open relationship, that would be purely sex, so she can explore and get some of those ?missed out? experiences. She also wants to share her ?new body? with new people since she was always the ?fat girl? for the past few years. I?d be lying if I said I didn?t feel sad and uneasy from this, however, I did emphasize with her. We agreed we would research this together before fully committing.
In the upcoming weeks, I did a deep dive into the idea of an open relationship. I was reading a few books and researching online. I decided to bring the idea about boundaries up with her. A few of those boundaries were that friends and coworkers were off the table (at least in the beginning) since it?s been known to cause a lot of friction. She seemed to feel a bit hesitant about this and would come up with ways or excuses that a coworker may work (ie what if this coworker was out of state, or what if it?s an ex-coworker). This bothered me a lot of and I asked her if there was someone specific in mind / why was she sooo attached to the idea of having sex with a coworker. She said there was no specific reason but to be honest, this didn?t sit right with me. Within the next week or so I would bring up boundaries again and we?d get into similar arguments.
Finally, I sat her down one night and told her that I needed her to be straight forward with me, and long story short there was a specific person she had in mind when she wanted to open the relationship. It was a coworker that she found incredibly attractive and had recently been friends with. This coworker, however, did not know about this. This was purely from my fianc??s fantasy (from what I?m told). This coworker was fantasy and someone she developed a large crush for. She would have numerous sex fantasies and sometimes even fantasize about him in bed while WE had sex. This felt incredibly deceiving and I felt like my trust was broken. We talked and argued about this for next month or two. Even though she never ?cheated on me? I feel like my trust has been broken. I?m now completely turned off by the idea of an open relationship and it?s put a strain on our relationship since.
Over the next 2 months we would talk... a lot. Through us talking she said that as we are getting older she's wanted to experience the single life and meet new people before ?it?s too late and she?s too old.? (This also includes other things that she didn?t get to do like partying at college and living alone). buuuut doesn't want to give up our relationship and wished we met later in life.
She left on a business trip about 3 and half weeks ago, where she will be away for another month. Before she left, she was saying how she was excited to live on her own for once (she lived with her parents and a roommate prior) and that it would give her some time to think about all of this and even a bit of the ?single life? experience she wanted since she can live alone. 2 weeks within the trip and she hates it and says how much she misses me and feels lonely and regrets ever thinking about about wanting those other things and everything that's happened in the past few months.
Honestly, the biggest thing is that I resent her for the way she's made me feel through all of this. I feel like I can't exactly trust her word or true intention and I hate that for the last few weeks I had to sit there and see if she'd enjoy living without me...
I feel like it?s been nothing but constant extremes and her desires for other things have completely pushed me away. I?ve lost a lot of trust in her and our relationship. I feel empathy toward her needs but it also feels selfish and hurtful especially since we are supposed to get married soon. I?ve been thinking about calling it off but I don?t know if I?m being too emotional / overreacting right now.
TL;DR ? Fianc? broke my trust when she lied about her true intention for an open relationship and her selfishness is pulling me away from our relationship even though she?s regretting the way she acted.


Source.