"He has of course sworn up, down and back again that she meant nothing to him." She meant enough for him to risk your marriage, YOUR sexual health (have you had a full screening done? Even if he did practice safER sex it's still important), hurting your DC...
I didn't forgive my ex and in your position I definitely wouldn't forgive.
I'm not much younger than you and being mid-fifties doesn't rule out another relationship, though there's a LOT to be said for being happily single too.
I think one thing you do need to accept is that you don't really know him, I'm sorry but you don't. If you did you'd have spotted SOMETHING I wasn't married nearly as long just 10 years and I noticed immediately the change in my ex (OW was a supposed mutual friend too so I noticed a change in her too). I stayed long enough to get ducks in a row and gather enough evidence he couldn't really deny it (he still did! It was ridiculous! Even when it turned out she was pregnant and baby was conceived 2 months before I blew everything up, she didn't know till couple weeks after I threw him out and he lost the plot. Wasn't what he had planned AT ALL)
There's a "thing" on mn called the cheaters script and it's amazing how many of us have been "read" this script!
Seems you missed much of it but at this stage it generally goes along the lines of:
Deny deny deny
Only admit what can be proven
Say "it meant nothing"
Gaslight cheated on spouse
Make cheated on spouse feel guilty for the affair happening/not "getting over it" in a time the cheater thinks reasonable.
He's lied to you repeatedly, deceived you about who he is and the nature of your marriage, treated you disrespectfully, risked your sexual health. He didn't even tell you and if you hadn't found out in all likelihood he wouldn't have and would have carried on the affair.
I don't understand how anyone can forgive that myself.