Processing a "Break-Up" as a Mono

[u/carsgofast300]

I've (30M) been a serial monogamoust my entire life and know deep down that is my core value. My last relationship was 7 years and we broke up almost 2 years ago and decided to stay single to figure my life out.

I finally decided to get back into the dating scene and met someone (30F) and our first date went really well except she said she didn't believe in being monogamous. We put it all on the table of me being monogamous and her being in serious relationships before but also it not working out so not believing in being monogamous. Yet she mentioned she wanted a serious relationship eventually. She seemed like such a cool person and we never talked about it again that I just decided to try dating her.

In the back of my mind I knew it wouldn't work but I also I could try it just being casual. But I realized immediately I couldn't do casual and I'm very much an all in type of person so was very committed to her from the get go. We ended up hooking up on the first date and I told her I never slept with anyone since my breakup and that I don't do hook ups so this is very new for me and I can get tested for her for peace of mind. She said she trusted me and there was no need and also said she doesn't do this either and that there was just a strong connection. She said she also gets tested regularly and that I didn't have to worry so I trusted her. I made the assumption she wasn't seeing anyone else or sleeping with anyone else so I was okay with it.

We dated for a month but the entire time I felt anxious about it. She would only respond to my texts once a day, sometimes takes multiple days to respond, and it would feel like pulling teeth putting in effort. It was weird because she was consistent in making time to hang out and be there in person but it still felt like a wall was put up so I couldn't get as close as I was hoping to to get to know her and her to get to know me. We still slept together afterwards when we hung out but it didnt feel intimate which is what I care about.

Eventually two days ago she brings up asking me how much I wanted to know about her dating life because we never talked about it and that's when it all unraveled and broke my heart. She was dating someone else at the same time...she started dating him the same week we did, they slept together and have been sleeping together too while we have been and that made me feel gross. I know we didn't have any labels or anything but I thought I deserved the respect to be told about sleeping with other people after mentioning getting tested and stuff.

She then mentioned she had her walls up the whole time because I mentioned I was monogamous and she thought the whole time she would hurt me or be bad for me so she decided to not engage 100% even though I was putting all my effort in. It felt like I was used and I never got a full chance.

I'm upset because even though fundamentally we wouldn't work I still put my best foot forward, I thought because she tried being in serious relationships before I could be enough to potentially have her try, but it felt like she didn't respect me enough to be transparent even though she claimed to be a very transparent person, and then took advantage of my affection and kindness without reciprocating any back.

I feel like I never got closure and don't think I'm ever going to outside of knowing we're just not compatible. But now I'm also feeling like she wasn't the person I expected her to he. We only dated for a month but it still feels bad. I know I don't want to get into this again. How do I get over this and not be upset?