I hear a lot of loss in your feelings, but what would help this is firstly an acceptance that you can’t change him and his choices, if he doesn’t stay with this girl and move to Brighton, it’ll be something else similar. It obviously feels horrible but there’s nothing you can do.
Then, having accepted this, you start making positive plans for you. Life is hard right now, and you might not feel like anything much, but you need to indulge yourself and pick up something new. A hobby, night classes, a new pet… if you’d like to make connections and eventually, friends - lovely grown up company whose own kids have moved on, or who have never chosen parenthood to start with, you could look into local meet-up groups, walking clubs etc. you could go on art holidays or volunteer with animals if that’s your thing. The important thing is that it isn’t more of what you already do - this is your opportunity to be more fabulous, more self indulgent, and to find out who you are as well as a mother, now you haven’t got to dedicate all your time and attention to motherhood.
Yes it’s a loss but losses make space for new joys to enter our lives. It starts with letting go, though. You’ll always be there to come back to, but it’s time to leave him to it.