I'm so sorry for everyone struggling with the "why did he do it". Unfortunately it's because he could and he wanted to, and as @Candlesburn says, he probably checked out a lot time ago and processed all this stuff we're going through long ago, he just didn't tell us as his partner.
Breaking up is hard to do for both sides, obviously worse for the person who wants to continue, and ideally the person whose feelings have changed does it with compassion and care but sadly that seems v uncommon. In my case my ex was unable to express his emotions, refused to discuss disagreements, sulked instead of discussed, so it's actually very on brand for him to dump me by starting a relationship with someone else.
My advice in the early days is much like others: be nice to yourself, connect with others, get counselling. The faster you re-establish your newly single life and start doing things for you, the better you'll feel. What did you not do because he didn't like it? Do a lot of that! Any places you didn't go, or people you didn't see, or hobbies you didn't start, or things you didn't wear? You are now free to step out of relationship compromise mode and into doing whatever the hell you want. And yes, it's scary at the start when you're used to making joint decisions or always thinking about someone else but I promise it gets easier as you practice making your own decisions.
I was driving myself home the other night in the car I recently chose and paid for, and I thought briefly of some ick things ex did, and I just laughed and shouted "fuck you Dave" (other men's names are available), turned my music up and drove on ahead towards my own bright compromise free future. Sadly I'm still living with ex a year on while we sort out housing, and I'm sure I'll have another dip when that's finally resolved, but I feel ok about it, it'll be briefly hard but I'll get through it and out the other side. I feel like I've got my emotional toolbox sorted, because of counselling, friends and me being nice to myself!
We are now free, our exes have done us the favour of releasing us from ever having to expend our precious energy on them, and we are responsible for our own happiness. Go get it! Xxx