He then met someone else and seems to be completely different, he treats her well and if anything she seems to wear the trousers in their relationship
You don't see what goes on behind closed doors, though.
People thought I 'wore the trousers' in my previous relationship (and in some ways, I think I probably did) but in actual fact I endured three years of serious abuse and violence.
My ex died a few years ago and when he did, there were dozens and dozens of tributes to him on social media, including lots from women, saying what a lovely man he was. And sure, he would have seemed like a lovely man to them, I'm sure, because they weren't there when he was hitting me, accusing me of sleeping with my own brother, smashing up the flat, emptying my bank account, threatening to torture my pets to death, threatening to burn down my parents' house, etc.
I was actually kind of in the reverse of your situation, because when I was with my ex, he was (and remained until his death) really friendly with his ex-wife, the mother of his child. She was incredibly nice and absolutely lovely to me and I liked her and her family a lot; it was literally never awkward and honestly some of the nicest things I remember about that relationship were the times we were invited to things like family barbecues etc at her and her husband's house. I used to wonder, every single time, why the hell my ex was abusive to me when he obviously hadn't been like that to his former wife - if he'd abused her like he abused me, surely they wouldn't have stayed friendly? I genuinely thought he'd changed between being with her and being with me.
Subsequently, I found out that he had been violent towards her too. His son, who I loved, once let slip that he remembered seeing violent fights between his parents when he was little, and I also found out that my ex had treated her like shit while she was pregnant. When she went into labour he drove her to the hospital, dropped her off on the doorstep and then drove off to the other end f the country to spend the weekend surfing, without telling her where he was going.
So, basically, you just don't know what's going on in people's relationships, and I doubt your ex has changed one bit.