Sorry this happened OP. Sounds similar to a situation I went through, except I was the "other" /ex. Sort of. Kinda.
Got with a guy, who had recently left his girlfriend. Chemistry was intense, he totally love bombed me, I thought he was dead into me and "the one". He told me I was perfect for him, wish we met earlier, had strong feelings for me quickly, obsessed with me, yadda yadda.
He said some awful things about his ex girlfriend alongside all this (how she was malleable, had nothing in common, stupid and simple were also words from him) , which should have been a red flag. I was an idiot though.
Soon, he started to act strangely. Controlling? Clingy? Intense in a bad way? Wanting me to stay at his place, despite the fact he still had his ex living there whilst she was trying to find a place (I said no, as it was disrespectful). He started to seem to want to play happy families, but it was way too rushed.
He left me - it was sudden, I think I'd said "no" too many times and he had lost patience. He told me he wasn't over his ex wife (not the ex girlfriend, the wife from before even that) , and wanted to try and sort things out with her. I was crushed. I may not have wanted to rush things or whatnot, but I very much liked him. I thought I loved him (I don't think I did, I was just caught up).
Anyway, not two weeks later, I see he's back with the ex girlfriend. Not the wife, the girlfriend. I was so confused.
Mutual friend, more his friend than mine (think he felt bad for me) ended up telling me that he'd tried to get back with the ex wife, she shut it down immediately, so he went back to the ex-girlfriend also immediately... Because he needed someone to help pay the rent (apparently that was his main reason), be a stepmother to his kids when he had them over, and he was tired of cooking his own meals/chores/laundry. He fed her a wild tale of how he regretted it, realised he loved her, etc (and then complained to his friend after about how things had turned out).
His own friend thought I had a lucky escape.
He's now marrying the girlfriend. I can only hope he changed for her. But I doubt it. He was already nearing 30 when he messed us about before.
I think some men just do this shit... They need "ol' dependable" in their homes... They'll marry to get a live in maid/nanny(/mother), but pine after others. And cheat given half the bloody chance.