@Gnarly999
You can't 'fight' for a marriage if the other spouse has no interest in fighting alongside you to change things. If he's determined to end things, then things will be ended. You can't stop him from divorcing you if that's what he really wants. All you can do now is 'plan'.
The best thing you can do now is go silent and see a solicitor. Take a 'snapshot' of family finances (salary, assets, bank accounts, the house, pensions, etc) and show the solicitor. They're in the best place to advise you of your position wrt settlement, maintenance, and access. It doesn't mean you're going to 'do anything', you're just educating yourself. And knowledge is power.
The issue of OW is neither here nor there, unless you feel it will light a fire of anger in you. I'm not saying it's no big deal, just that right now that's not what you need to be wondering or focusing on.
As far as working, you do need to take stock of your qualifications and ability to reenter the job market at a 'living wage'. Ask the solicitor if it's advisable for you to start looking for or to go back to work. YMMV but my cousin was advised NOT to change anything about her 2 days/week work schedule until after the settlement was signed, sealed, and delivered.
I know right now you feel you're in a tailspin. But you will get through this. It won't be easy and it won't be fast. This is a marathon not a sprint. So take your time and make your moves deliberately and after careful consideration.
Also, seek support. I know you haven't told anyone yet, but find someone IRL, a friend or relative, with whom you have confidence in their discretion, and talk to them. They won't be able to solve your problems but there is such a relief in knowing someone is there for you, even if all they can do is give you a shoulder to lean on.