Kindly... if your H is set on separation and divorce, there is not really anything you can do to stop him. My advice would be to take the separation agreement to a lawyer and review it. I was in the same position as you last year...my normally unorganized H got his sh*t together and presented me with an agreement that, TBH, was actually very reasonable and even weighted a bit in my favour. I know now that there was an OW motivating him to get it done but I didn’t then so the sense of urgency was a bit baffling.
Anyway....I honestly feel that if I had put up a big fight or tried to slow things down, the agreement would have been a lot less favourable for me and it still wouldn’t have changed the outcome. I would still be divorced. So....as hard as it is...you need to deal with this part of things as business like as possible and make sure you get a fair agreement. By all means, take your time and don’t go out of your way to hasten the process but don’t actively try to stop it either because the reality is, you can’t. It takes two people to marry but only one to divorce. At the end of the day, if he wants a divorce, he will get one.
I’m sorry you are going through this. I KNOW how hard this is. But... you CAN and you WILL get through this and you WILL be OKAY. I promise. It gets better with time. (((HUGS)))