@Waffletots
I'm reiterating what others have said; you really need to speak to a solicitor, sooner rather than later. Seeing a solicitor doesn't mean you're going to 'do anything', it just means you're educating yourself on what divorce may mean to you if it comes to that.
Get together a 'snapshot' of family finances; income & assets for both of you, including pensions, monthly expenses and debts. Just a rough idea will do for now. Tell the solicitor that you need to know what to expect as far as settlement, maintenance, and what child arrangements are 'usual' based on your children's ages. And take your dad with you. Since he's the 'money man' in the family his input and memory of what's said will be invaluable. Take the information you're given, write it all down, put it all in a folder and put it in a drawer at your folks' house for safekeeping. Again, this doesn't mean you're taking any actions now, or even any time soon. But you are in uncharted territory. You never imagined your H would cheat so you can't just tell yourself 'Oh he wouldn't ever play dirty'. Forewarned is forearmed. Knowledge is power.
Sorry if this has been mentioned before, have you informed his family of what's happened? Just a simple "I have asked 'Bob' to leave the house. He can explain why". Obvs doing this will depend entirely on what reaction you think they'd give. Don't do it if you'll get a ration of shit or if they're the type to tell him "It's your house too, go home".
You are doing so well. I know you're feeling things you never thought you'd feel. But despite that, you are holding your own. You can do this.