Try to remember that the very fact that he can do this means it was neither what it seemed to be with you, nor is it what you imagine it to be with the new person. There is something else going on there. It is a performance for himself, if that makes any sense.
Sometimes the proof we need is staring us so big and bold in the face that we cannot even see it. He wants a simple answer. He hurried up and got one. Now what?
It devastated me when my ex didn't feel compelled by the loss of our r/s, that he had said was so special and important, to dig in and do any meaningful work on his issues in relationships. He just went and got a new one. Right away. It was excruciating -- constant dreams about it, couldn't sleep, constant preoccupation about their happiness. Why would he make all these accommodations and changes to be with her, when all ("all" I was asking him was to look within & figure out why he implodes relationships?
I spent months worrying about this. I was still worrying about it two months after he left her. (I did a really good job of no contact

and had no idea they were over.)
I know he is still looking for that perfect r/s where nothing will ever hurt or be scary. He will not find it, but he will continually divert himself with the possibility that it is right around the corner with some new woman.
I'm sorry it's so hard, but I promise it doesn't mean what it feels to you like it means.