Why do they act so cold in the end?

I’ve been seeing this man for a year and a half, and things have been far from perfect, but one thing that I always knew for sure was that he was a kind person and that he cared about me. He denies it, but it’s as if he’s turned into a completely different person in the span of a few weeks. He was usually the one asking me why I was so negative and telling me that we could make it work, and now he seems hopeless and cold and distant and cruel. I get that maybe he’s checked out, or maybe he’s seeing someone else, or maybe this is who he’s been all along, but it just feels so wrong. Where is the person that I’ve known for the last year and a half, and why does it feel like he’s gone forever? He says that he cares but he just seems so cold. I don’t get it.