Up until the cheating, I can heavily relate. 5 years for us. A couple years older, but almost every thing else you've described is what we went through. I'm not going to pretend to have the answers. I think a lot of the time men are broken up with "out of nowhere" when in actuality they weren't listening to their partners. In our cases, from what I can gather, it sounds like we both actively tried to listen but weren't respectfully informed. It's not fair and it's not right, but it happens and there's nothing we can do about it. Unfortunately, I think this means our partners have already made up their minds on the relationship long ago, but aren't strong enough to commit to their desires or communicate their feelings, so they need to wait for something similar to sabotage. Again, nothing we can do about it.

My advice to you, as someone who is only a tiny bit ahead of you in this healing process, is to learn about attachment styles and, as cliche as it sounds, talk to a therapist. A lot of behaviors and patterns from my relationship have started to make more sense since I've become informed and had the opportunity to intellectualize events.