Similar happened to me. Years of total bliss; people would literally ask us how we had achieved such a fairytale long-term relationship. This was my dream man and we were talking about marriage (we had started dating at 18 so, at 25, we felt we were finally getting old enough to go ahead and do it).

He left home one morning totally normal - kisses, breakfast, inside jokes and laughter, "see you after work" - and then never came home that night. His phone was off. I panicked all night. Finally received a very angry call at 8AM - he was in jail for a DUI (never had drinking issues before). I bailed him out, got him a lawyer. He was angry, cruel, the opposite of who he'd always been...I thought it was extremely weird at the time, as he's the nicest guy ever, but I thought he was just stressed about these events. I had no idea it was only just beginning.

He bailed a few days later, periodically sent me crazy texts and messages ranging from lovey-dovey to downright cruel, then gave me the silent treatment for nearly 2 years, and when he popped back up he was dating a girl who shares my birthday, all of my interests and personality traits, and looks just like me (except...a way uglier version. I feel horrible saying it, because she truly seems like a LOVELY individual with a great personality, but...seriously. You really can't miss that I would be EXACTLY her if I got hit a few times with the fugly bat). He asked for me back, communicated with me pretty obsessively for a time, but he was also sorta hanging onto other gf too....it was some weird shit, but irrelevant anyway, as the trust was CLEARLY annihilated permanently anyway. Also, his personality is quite different from who I knew - a rather lower caliber, honestly. Cool with lying, etc.

Still no idea what happened. It tore me up for a few years but eventually I had to stop trying to understand because it was making me looney tunes. Best I can estimate, he just flat had a mental breakdown. No idea why. I have an amazing dude now so it worked out well. :)