Op surely you can see that it isn't his money??
You gave up some of your income, and presumably pension, to bring up your mutual children. You took a financial hit, which allowed him to keep earning. Without you, if he'd had to look after his children, he'd have less money. You are married and, in law, all money and assets are jointly owned. And aside from the legalities, morally they are jointly owned as you are a couple. You've no doubt supported him in his career. I'll bet you've parented your children, kept the house, cooked, cleaned, done his life admin, looked after him. Which allowed him to focus just on his career. So the benefits of that are morally and legally jointly shared.
And then we come to how he's treating you - you owe him nothing. No concessions, no compromises. Just take what is legally and morally yours - half of everything. Quite frankly he'd be very lucky to get that, as he clearly doesn't see any moral obligation to you and has treated you shoddily.
He wants you to stay around, presumably so he has a housekeeper, so he has a comfortable life, so he doesn't have to 'lose' half of 'his' money, so he can pretend to be a respectable family man, so he can still see his sons without making any effort.
You would be so much happier without him. And you could be a better mum to yours sons if you weren't with him. And you would show yours sons how to act when someone treats you like dirt.
By the way, it isn't ethical when one person decides to have an affair or many affairs - it's plain cheating. He wants you to say you agree to it so he can feel better about himself and kid you and anyone else that it's all good and he's not been a terrible person. He's lying to you. He wants to cheat, and for you to remain his housekeeper and give him all the benefits you currently give him. He needs you more than you need him. He gives you absolutely nothing. You've given him the bulk of your life, your earning potential, your self-worth.
And please don't assume all solicitors are bad. That isn't my experience at all, and to suggest it is very unfair to the many good and honest solicitors out there. Try one, get some free or cheap initial advice and see where you stand.
You deserve better.