You poor thing 💐 It's good that your mums coming over so you can get some rest and be looked after.
My exh did this, about 6 years ago now. They're still together and have just bought a house. It's actually fine, I have a boyfriend who is amazing and a good relationship with my ex as we share residency of the kids and he's a great dad, much better now than when we were together funnily!
The partner of the OW (at the time) was his best friend, they'd known each other since they were kids. He's also moved on and is happy with someone else. Occasionally we all socialise together, all water under the bridge now but at the time it kicked off massively and lots of people got hurt, but time does heal, it really does.
I guess what I'm saying is yes it's painful and awful now, but your life is not over, it's just gonna be different now. Your future no longer looks like how you thought it would and that's a huge shock. But things will be ok, one way or another. You will get through it, I promise xx
(And move quickly with the legal side of things, while he's still guilt ridden)