I was engaged in my early twenties and the way he chose to break up was to marry someone else two months before the wedding and let her tell me. We were in a long distance relationship at the time because of his work, but had been together eight years. I had no clue whatsoever he was seeing someone else. She dumped him three months later and he wanted a second chance with me. I never spoke to him after the initial break up. I don’t know which of your categories that would fall under, since we never had the fight and it certainly wasn’t civilized and amiable. Perhaps it could be called an indiscretion, but that doesn’t seem a strong enough word. I just knew I would never be able to trust him, ever, if I took him back. It definitely wasn’t easy though, and to this day I don’t know why he did what he did - not that it would have made a difference.
Looking back, I would have appreciated him being honest when he started seeing someone else instead of being such a coward about it. It still would have been painful, but maybe less so. So sorry you are going through this OP. It seems to be one of those universal things everyone goes through at some point in their lives. All I can say is it will get better with time.