OP, I could have written your post six years ago.
Twenty years of marriage, three teenagers, two of which about to take exams.
He no longer loved me, there was no one else, etc etc...
Both about to turn fifty.
Decided to wait until Summer holidays to tell the kids (this was April).
Couldn't cope with it, it was bloody awful. He became so cold and detached it broke my heart. The kids knew things weren't right.
Long story short, I flipped one night, packed his bags and chucked him out, filed for divorce the following week. Felt better immediately, because I had control.
Atmosphere improved at home immediately. Kids got through it, passed GCSEs and Alevels.
Other woman crawled out of the woodwork within weeks of me chucking him out. They'd been seeing each other for years.
Think there's OW here. As my SHL said "men don't give up what he's giving up to sit alone in a bedsit and eat Pot Noodles".
In a nutshell, teenagers know more than you think, you need to take control and put yourself first. Also, kids are incredibly resilient.