He has had a six-year affair, so you can take as long as you need to work out what to do next.
I'm the same age as you and it's hard to say what I would do in your situation. If everything else was okay, I might just accept that he wasn't the man I thought he was, but not be so bothered that I would be willing to uproot my entire life even more. Would I want to end up divorcing at 53, trying to find a small house, be thinking about getting old alone? If you get to 50 and your children have grown up, you sort of realise that there's more to marriage than sexual fidelity - if you are still together, there is still a home for the children (and grandchildren) to return to, and your life will run its course in a relatively predictable manner.
If you decide to end your marriage, it's a massive leap into uncertainty and, probably, relative poverty. You might have to get a job, and if you haven't had a job for a long time, that is in practice going to be as good as impossible (not an issue if you've always worked, obviously). Your children will have to navigate split birthdays, Christmases, weddings, etc, etc.
But it could also be the best thing you ever do. You are still young enough to do something radically different. Go and live abroad, or travel, or do any of the things you have always wanted to do but are difficult to do while you're a couple. Meet new people. Maybe you would meet a man who didn't want to lead a double life, and who thought he was so lucky to have you that he would never look at another woman again. Maybe you could meet a millionaire and never worry about money again.
It's all a massive gamble, but it's definitely not as straightforward as "LTB". And it's not a quick decision. Eventually your gut will tell you what to do, but don't make any hasty moves.