(((Big Hug)))
I'm so sorry this has happened to you. In this day & age it's unforgivable, he could have lived his life as a gay man.
even when someone has treat you very badly, it's hard to stop loving who you thought they were & not defend them to others, so I don't think us calling him all sorts of things is going to help.
I think when he said 'he'd try to stop' he probably meant it as he didn't want to lose you, the kids, his home life & all he knows, but soon realised he needs to be true to himself.
his MH is probably in a bad way, it's a big thing he's going through & as you've lived him for so many years. That's going to be hard to see him going through & not be 'his person' to help him.
but you absolutely must put yourself & the kids first now. Emotionally & financially.
talk to him & decide what you're going to tell the children now & when. Given it's now Sunday night, I'd wait until Friday so
they have a couple of days where they don't have to go to school. Personally I'd go with the Dad & I are separating, Dad loves you very much but he's decided he loves someone more than me & wants to be with them instead. Yes, I'm very upset but I'll be ok. It will change our day to day lives, but it won't change how much we both love you & together we can work it all out. You can ask us anything you want to, but we might not have all the answers immediately, somethings are going to take time to sort out. For now you two & me are going to keep living in this house & Dad is going to live at (wherever) but you'll still see him lots (presuming that's the case??)
something like that anyway. Id be honest about the 'who' if they ask. Imo there's no point in hiding it.
Same with family & friends. It cannot be secret. There's no need for bit be & being factual about it us jyst so much better.
and yes, you need to go & get fully tested .
Lots of love & support! It might not feel like it right now, but you WILL be ok. 🤗