I just need a little off-load, as I know I’m being unreasonable (and this isn’t AIBU).
Exh and I have been separated nearly 3 years now and have a 5yo DD. He was a bit of a shit partner towards the end of the ten years we were together; nothing really horrid but some of the “worst bits” include getting arrested for drink driving and getting a 13 month ban while I was pregnant (ok that one is bad), and using webcam sex chats. We also just really clashed/rubbed each other up the wrong way and basically fell out of love.
I wanted to be happy, my DD to be happy and I also wanted him to be happy too. After we split I met a lovely partner who is amazing, and yes, I’m really happy with him. Exh bounced around between a few short term relationships/tinder dates, that sort of thing.
However, he now has a more serious new partner (since December) and is moving in with her. She has two DC near our DD’s age, the kids all get on. Happy families.
I know it’s horrid and wrong but the whole thing just makes me feel so sad, and I guess inadequate. Like; why can he make it work with her when he couldn’t with me? Why is he interested in family days out now when he wasn’t with me and DD when she was a toddler? What did I do wrong, why wasn’t I good enough?
I know I know I know it’s very early days for them, and (not to drip feed) I actually know that he already cheated on her with one of his previous tinder hook ups after they’d been seeing each other a couple of months, and she knows and took him back, so it’s not all rosy. On the surface I’m happy for them, and I know this is all for the best; we couldn’t have stayed together, and I definitely don’t want him back. But deep down it churns me up thinking about why, why, what if and not fair 
Is this normal, will it pass?