The old adage is "men cheat because they think they can get away with it, women cheat because they're out of the relationship." Yes, it's sexist, but I think it's true here. I'm sorry, you're losing her. If you put your heart and your dominant male masculinity into it can you keep her? Yes, but it will never be the same. Sure, you'll have your companion, but so much will be overshadowed by this. It's so hard to comment on a relationship 25 years in the making, but you're asking, so I'm telling. You lost her - 2 options. Have a relatively loveless marriage or divorce and hope for something better. It's so hard, but I'm sorry, she's gone.