Re: how does guilt consume a wayward spouse?
I think it's very common to just 'forget' long periods of committed time with SO's when the party is over. For some people, when the love is over, it's over. They put the memories in some box that says the SO is 'someone that I used to know.'
Your ex was a dog with a bone before you proved his cheating. He wasn't letting go of you as long as you were willing to see him as a reasonable, respectable prospect. It wasn't you personally that he wanted - he just didn't want to lose something that he had. He wasn't giving up the bone. Once you knew for certain about his lying and cheating, he could dispense with this charade.
The guilt excuse is just another charade. As others have said, he doesn't want your relationship anymore, but hasn't been man enough to cut the cord. First he strings you along acting like he wants back in, and then hits on the 'guilty' excuse to keep you at bay once you appear to have bought all his previous crocodile tears.
A common coward's way out is to say, 'I've hurt you so much that I fear you will never get over it, so it's better that we split,' or 'I'm so consumed with guilt that I can't see us working together as a couple anymore.' These are just code for 'I really don't want to reconcile, but don't want to sound like the POS in all of this. I know that I burned the house down, but don't want anyone to know that I'm not willing to even try to rebuild it.'
The man's a coward and you deserve better.
And 51 isn't too old to 'dump a cheater and gain a life.'