My stomach is in a knot for you right now - I'm so sorry. This shouldn't have happened to you, and it's NOT your fault that it did. I know we keep saying that, but it's because we really, truly want you to absorb it and believe it.
The truth is, you won't be able to force it to make sense in your head because it just doesn't. Everything you're describing about the life you two had sounds absolutely lovely, and he has either reframed it or else is willfully ignoring it in order to allow him to carry on with this personality crisis.
I truly do believe it sounds like he snapped under the threat of looming adult responsibility (first your marriage, then kids talk) and it sent him into some kind of mental health crisis, and he's handling it in the worst way possible. The suddenness strikes me, too, and the fact that all the other people in his life are equally stunned.
Whatever happened - it's not your job to fix it, nor could you if you tried. I don't know what the future will bring for him or you - he may wake up tomorrow in a cold sweat, horrified at the mess he's made of his life, or he may never snap out of it and will remain this new, callous and cruel version of himself forever. The point is, at this moment, he is unreachable to you and as hard as that is to accept, you must accept it for your own health and sanity.
Your job is to take care of you now.
I have to tell you that, as hollow as it may feel to hear right now, you sound like a wonderful person. Warm, caring, fun, spontaneous, loving - and you're young. You probably don't feel it right now, but you've got your life ahead of you and not behind you. If there's one thing I've learned while being forced to eat my own shit sandwiches, it's that a life can change completely in the space of a single year - or less. And that goes both ways - it can completely fall apart (as you are feeling now), but it can also transform into something new and unexpected and beautiful.
If this truly is the end, then he is the one who has lost something here. And it's something that, no matter what the future brings, he can never get back. This is the bitter pill that even remorseful WSes who have reconciled with their BSes must swallow. One day, one way or another, he'll have to taste his medicine.
But again - not your circus, not your monkeys. Not anymore. He doesn't get to have you at his side anymore, and damned if that isn't HIS tragedy even more than it is yours. Even if he doesn't yet realize it.
You'll make it through this, and you WILL be happy again. You can't fathom it now, but one day, you will.
I'll light a candle in my mind's eye for you, and I wish you every bit of good karma the world has to offer.