I do really sympathise. When my DiL became pregnant for the first time, my DS said, jokingly (or was it??) that I could retire early and look after the baby full-time instead. I responded in kind, laughed my head off and said "yeah, you wish!" or something similar.
I then went on in a more serious vein to say that I would be more than happy to help out with babysitting now and then and in emergencies, but I wouldn't be prepared to commit my time to a permanent full-time (or even part-time!) child care arrangement. So we all knew where we stood right from the start and it's worked well. We love having our GC and spending time with them, but even the odd day or two here and there is exhausting!
I am continually shocked at how many parents expect the grandparents to provide free full-time childcare, and seem to have no clue about just how exhausting this can be to people in their 60s and beyond. Utterly selfish in my opinion, and rude beyond belief to expect them to pay for all their food and entertainment on top of this!
I really feel for you, because now you will be worrying about how to get your relationship with them back on an even footing. But please, please, do not torture yourself with guilt. The only people in this case who should be feeling guilty are your son and daughter in law for taking the piss for so long.
Incidentally, I don't really understand how you were doing 80 hours a week. Serriously? That's 16 hours a day over a 5 day week! Surely not?