Full claps for the honesty, art and writing, but I'm going to go against the grain of other commentors.
Your relationship with your ex is wildly unorthodox, when you were married and after separating (are you divorced?) Speaking from a man's POV, a husbands POV, I don't believe your husband is fine and all glowy with this new life. Maybe he is.
For me? Marriage ends, okay, it happens. Be happy for the other moving on and finding happiness? Sure. I'm not a monster.
But to see it? To think I'd want to hang out with Mr. New Guy and watch your feet in his lap and your chin on his shoulder and become new best friends? No, fucking thank you.
I just can't see how that's rewarding, fulfilling, or peaceful for him. And you would like your ex or soon to be, to be fist-bump pals with the new guy while keeping the old shoe around because it still fits and feels comfortable? I don't believe that's sustainable.
But then again, I am an old-fashioned guy. I mean, if my wife split and found new love? Great. But I don't want to have my face in it everyday.
I mean no ill will and wish you all the happiness in the world but I'd expect if your ex was honest, his feelings are not on the same wavelength as yours.
And you said 'you're not sure what to do with those memories-feelings.' Well, if you care even a tiny bit about your ex, please don't sit and reminisce with him.