@Lookslikelou
What do I do? Our marriage is over but how do I cope?!
What you do now is focus on yourself and behave with dignity. You'll be glad you did. No emotional or angry scenes, no 'pick me dance'. Calm, determined and dignified is the way through this. Remember also that the opposite of love is not hate. It is indifference. You may not feel that now, but fake it until you make it.
The issue of 'grooming' or whatever will have to wait until another day. I'm not saying it doesn't happen, but there are cases of young adult women becoming 'involved' with former teachers, coaches, etc that don't involve grooming. This young woman appears to have assured your DD that there was no grooming and that she is OK. For now, take her at her word.
At any rate, you have other fish to fry right now.
So first thing, see a solicitor to get the 'lay of the land' as far as divorce goes. Seeing a solicitor doesn't mean you have to 'do anything'. It just means that you are educating yourself as to what divorce may mean to you, financially. As both of your DC are older, I assume child access and/or maintenance don't come into it. But you need to get a financial picture wrt the marital home and any assets you and he have, either joint or separate. Take a 'snapshot' of these things with you and possibly another person as a 2nd set of ears. I can't stress how important this is because forewarned is forearmed. Things may be quiet right now, but men often turn nasty once they realize you aren't going to welcome them back with open arms and forgiveness. And especially when they realize that a divorce may cost them, sometimes dearly.
Also, change any passwords of yours that he has access to; email, websites, credit cards/debts, apps, banking. If you have joint banking, consider strongly separating your finances right away. Where I live we are advised to take only half of any joint accounts regardless of 'whose money is where'. A solicitor will advise you the best way to do this. But you need control of your own finances, right away. Again, their 'contrition' usually only lasts so long. Be prepared.
That's the practical side of things.
As far as how you cope emotionally, lean on your family and your friends. Beware of leaning too heavily (if at all) on your DDs. He may be a shit but he is still their father. They deserve to form their own opinions of his behaviour and determine their own 'path' as far as a relationship with him. I'm not saying that they both shouldn't know, just that if they decide to maintain some form of relationship with him, again, calm and dignified is the way to go.
And consider counseling or therapy. Having a trained but emotionally uninvolved professional to help you sort things through can be of immense help. You can pour all your hurts, anger, and feelings on them and they'll help you sort through things and 'put them where you belong'.
Finally, remember that this is a marathon not a sprint. It's going to take time but you will get through this storm and emerge with all flags flying.