Yes, my husband started saying shit like this when he hit his mid-40s and was going through a midlife crisis. He kept dwelling on the past, re-thinking all his decisions, romanticizing all the coulda/woulda/shoulda stuff that probably never would have happened anyway. Of course it couldn't be HIS fault that he had missed all the good opportunities and failed to live up to his potential, so it had to be MY fault. Marrying me and having a family was all a huge mistake.
He also criticized my body and clothes. He said I wasn't the fun and spontaneous girl he married. Imagine that! I had spent the past 20 years raising our kids and managing our family almost single-handedly. And yes, I had also gained weight, which was absolutely a fair topic of discussion (I've since lost 50 lbs), but it wasn't enough cause to toss a 20+ year marriage. It was really just another excuse to take out his personal unhappiness on me. Like your husband, he told me that, once the youngest was out of the house, he was leaving.
Here's the good news, after spending about a year flirting with divorce, he eventually came to realize that he was acting like a complete ass. He made peace with himself and his life choices and, after a lot of hard conversations, we got back on track. That was 2 years ago and things are really good. I hope your husband gets himself figured out, as well.