Welcome BluSTL.
As a veteran and fellow neighbor, I want to offer you some sound words of advice. I want you to know right off the bat that I am a straight shooter, and don't paint my responses with rainbows, and flowers, so sometimes it comes across a bit harsh.
Your H is having some sort of midlife crisis it seems, and it's time to stop letting him run the show. You need to take control, and start getting your feet under you again.
1. See a lawyer ASAP. Find out your rights, his responsibilities, and learn how this shit will play out, and if it matters who files, and how long it takes and all that.
2. See your Dr, get a full STD panel including blood work. Talk to your Dr about why, and if you are having trouble with sleeping and eating, let them know. Many of us needed a little pharmaceutical help to get us through, and malnutrition and sleep deprivation make it much more difficult to keep your head in the game, and keep the emotions under control.
3. If he is serious about staying, and really R'ing then he should have ZERO qualms about sending the Bunny Boiler (fatal attraction reference used here) a Cease and Desist letter, penned by him, or your family attorney. She needs to know that if she doesn't back off you are going to move forward with charges, restraining orders, and all that. If he balks, serve him.
4. Lay down the rules for what you absolutely require of him to R. If he balks, serve him.
5. Quit listening to the stuff he spewing from his mouth. The one thing you know is certain right now is that he is lying liar that lies out his liar hole. Tell him you don't believe him or trust him, and it's his job to prove through actions, that are consistent, and compassionate, and loving that he really does want R. If he balks, serve him.
None of us that were successful in saving our M's did it because we were the nice guy, or we let them take control of the situation. We managed to get through it because we got the light switch to flip in their broken little brains that they have messed up, and messed up BIG. That they have tons and tons of work to do to fix their broken selves, and then did the work.
Do Not allow him to blame, minimize, or rugsweep any of this. YOU and your children deserve much more, and if he can't give you that, then you don't need him, and you will be better off, and certainly much healthier without him.
(((and strength)))