Year and a half wasted.

So I had a man chase me a year and a half ago. He told me he was recently out of a relationship and they haven’t been good for years. She told him 2 year prior that she didn’t love him anymore. On top of that, she had cheated on him multiple times, never cared. He had to go to the hospital once and she went out drinking instead of being with him. They have three kids together on top of it. I decided to go out with him. Small things would bother me about them getting along with one another. He told me they were being nice for the kids. We talked about marriage. He picked out a ring. He told me I was the best thing to happen to him. Well three weeks ago she called him about getting back together. He told her that they have moved on and he was with me and things were good. Well I guess they talked two more times after that. I got pissed. I told him off. Told him it’s her or me. Well he told me he was choosing himself. He said he wanted to be single and work on himself. Well 6 days after we broke up, he posted on Facebook how happy he is to get his family back and how happy he is. Like I was nothing. He told me I was the one. He never had anything nice to say about her. He always said I was perfect and the opposite of her. How can he do this?