Is this normal?

Why is it when you see your ex moves on all of a sudden you want them back or something ? Or not even necessarily wanting them back just missing the things they used to do for you and realizing they’ll be doing it for someone else?

My soon to be ex husband and I had a very toxic relationship. I left him and have been separated for almost a year. He’s finally met someone and she’s around the kids too, which I don’t mind. I get to meet her. But I’m hurt and I shouldn’t be. We were together for 7 years and had 3 boys with our youngest being 8 months old. It’s just hard I guess. I can tell he’s really over me too. I don’t want him back even if he said he wanted to but it hurts knowing she will get some of the good things I got. Hopefully she doesn’t get any of the bad things I got. I wish them well, truly.