aw, i think it's lovely that you've been there for your kids, your grandson, and husband. twice a week sounds very doable on the surface, and clearly it was okay for a while.
but now it's not. for whatever reasons, and going back to work is a biggie, it's no longer something that works for you.
i'm so glad to hear that your son is grateful for your help, and does pay for your gas. but it doesn't sound to me as if it's a strictly money issue, more that it's exhausting you. i get it. while this wouldn't be a burdensome schedule for a young, energetic woman, we DO wear out more as we age, and simply can't handle all the demands that we routinely aced in our youth.
even if you gracefully withdraw, your grandson won't be in full-time daycare (unless the other grandparents are also stepping back?) being in daycare a couple of times a week won't hurt him.
i think you need to have a loving, forthright conversation with your son and DIL. all this guilt and worry is having a toxic effect, not just on your enjoyment of the baby, but your reaction to the suggestion of a sleepover indicates burnout. i'm sure your son didn't suggest it to weigh you down further, but simply to attempt to make things easier on you by sparing you the morning drive. he's trying to be helpful. it doesn't work for you. that's okay. you can say so, with honesty, and with honest appreciation for them trying hard to make it more doable for you.
i get that it's hard to have that conversation. but do it. do it lovingly, and without sounding accusatory or judge-y at them, and without apologies and guilt and self-blaming either. it's something you'd like to do for them, it's something you HAVE done for them (you've given them over a year! i wish my babies could have had twice-weekly grandparental care for a year!), but something you can't do any more.
no need for guilt, or remonstrances, or blame. it's just how life works. situations change, and we all adapt. your kids, and grandson, will too.
khairete
S.