I'm depressed

My ex treated me wrong so I don't know why I care, but I do. He was nasty to me every day and I still wanted to be with him, partly because I ignored the millions of red flags and only focused on the good times, and the other part of me thought I'd never be good enough for anyone else.. He made me feel that way. He got into a relationship days after we broke up and he kicked me out of his house with our daughter and my son from a previous relationship. I have him blocked on social media but can't completely cut contact because of our child. A number of things have confused me because the girl he's with now is the exact of what he always said he hated, she has a full arm tattoo and he hates tattoos, she smokes and he hates smoking, he also gotten a cat with her and he absolutely hates cats with a passion. I only live around the corner from his house so I have to pass his house regularly whenever I need to go anywhere because his house is on the only route my street leads onto the main road, so his house is unavoidable pretty much. Even though I don't want him anymore and I'm happy that I got out when I did, I can't help but feel sad. Why has he chosen a person who has all the qualities he doesn't like? He was with me for 8 years and he's just gone with her after breaking up, so soon. Makes me sad that's all. They've been together for 4 months now, so our breakup is just as fresh as their relationship.