Background: Me (31) and my partner (29) came back together after a 7 month breakup back in april. The reason for this first breakup was that i was in a bad mood in terms of anxieties and snooped over his stuff while he was not home (at his place). After going no contact for that 7 months, we randomly ran over each other and realized that we were both not over the relationship and still loved each other deeply. So we came back together. Everything was super great and we just felt like we were alive again.

The issue: Starting from the beginning of august, he had to move to another country for 3 months for some academic program and we decided that we better try ethical nonmonogamy as we both found this the right thing to do and we both wanted to try it for ethical reasons. We discussed on which grounds we should do that (whether to know any relation immediately or talk about these once he returned back etc, also talked about my anxieties to try etc) and decided that it is good that we move step by step by talking each case when it happened. After a month he had gone, we had some communication problems due to some conflicts. I also visited him for a week there on mid October (this was preplanned), which was also not very pleasant due to the ongoing coldness btw us. And he didnt write anything to me while i was hitting the road across countries to visit him, not a single word, which was also heartbreaking for me and I sent a resentful message about this as well. During this 3 months, i have never been with someone else as i didnt want to and thought he didnt as well as he didnt tell me such a thing.

He finally came back at the beginning of November. I went over to his place. And while having sex, i learned that he had sex with someone on september. I got mad that this was contrary to our agreement and he said that he thought we agreed to talk about these once he returned. I left the house and told him that i wanted to breakup. After 2 weeks of fighting through messaging etc we decided that we sit down and calmly talk about the problems, the mistakes, the possibilities to recover the relationship that was sinking. This was a good talk where he admitted his mistakes and me also mine, he apologized that he was behaving avoidant when he was not here, but we agreed that we were the love of each others lives so we wanted to fix things. We again felt that warm and cuddled and at some point started having sex. I had the gut and started asking what was it like with the other person, etc. I now that these are not good to talk about during sex but i couldn’t help it. He said it was regular not like ours. I asked how many times did he had sex and he said twice. I asked whether it was with the same person and he said no the other with a different person. I was surprised that there was another and asked when it took place. He responded that he couldn’t recall the exact date but was somewhere over mid or late september probably. I didnt believe and asked him to admit that it was indeed the night i was having a flight to visit him and when he ghosted my message. He couldnt say a thing, only that he was going to tell me but the situation btw us was so fragile etc. I got total disgust towards him, slapped him that we just had the same convo 2 weeks ago and he should be kidding me. Insulted him, cried, told it was over for good, at this point we cant even be friends because of his disrespect and eventually kicked him out of the house and blocked him everywhere. He was just able to say that he wants to fix things but i was super super angry and felt like being fooled around. Felt like this was actually cheating.

This was 10 days ago and it has been super complex for me to understand since. Sometimes i feel like it was meant to be over, sometimes I feel totally disrespected, but all the time I miss the love we had because it meant the world for both of us. I am feeling super depressed about my broken trust, sometimes think although it is pretty shitty and trust-breaking whether this is a complexion in trying nonmonogamy the first time etc. My friends are all monogamous and i feel like sometimes they dont understand the nuances about this form and comment pretty normatively. I am broken i am sure about this, but sometimes wonder whether this is fixable although it is a big big mistake. I realize my own mistakes in behaving so furiously and regretting this afterwards, or just immediately leaving once disappointed (i am sparing the last case). I am pretty confused and want to hear about those who struggled with their partners in nonmonogamy, whether you achieved to overcome or was it a mistake to try.

Tl;dr: omission of telling the truth in a nonmonogamous relationship which was long distance for a while, fixable or not?