and he won't let me see the messages they send to each other.
This means it's an affair - emotional, possibly also physical. It's rare men will continue an emotional affair for long without it becoming physical but certainly secrets like this do not a healthy relationship make!
I've never met her
Why on Earth have you never met her? In 18 years of marriage and I'm guessing you've been together over 20 years?!
Sorry to ask this, but is it definitely her he's talking to and not someone else? He could be letting you think it's her because you might be less suspicious of him talking to someone you already know is a friend and that's why he won't let you see the messages
Also a real possibility - using her as an "alibi"
I would have always said that he isn't the type to do this to have an affair
That's what I and my exes family and close friends thought too...
Then he knocked up the ow and they're now married with 5 kids!
Screenshot the messages and send to your phone/tablet in case he deletes them.
I would also advise "ducks in a row" time - gather all legal and financial info and paperwork, get legal and financial advice and start getting organised for a potential split - even if you think you don't want to or that he won't leave you, you need to be prepared and it's a way to use your anxious energy in a constructive way.
Don't speak to him yet, wait until you're a bit calmer.
So sorry you're going through this op 