Please help me see where I am going wrong.
Well that is where you are going wrong, - you are blaming yourself. You have as you say, a difficult daughter. As others have said, stand back, do less, get less involved. You are doing nothing wrong.
Do you have any outside interests, do you and your DH have a life together, involving the two of you doing things you enjoy.
What you need to do is ration your daughter and DGS. Limit your texting to just telling her things she needs to know. Do not bombard her for texts. When her husband is away. Invite her round to your house and make it clear going places is not on the agenda. Have some, toys, not too many inyour house for your DGS to play with, pens and paper, a few books to read, some lego. That is all that is needed. Other times you could go out, but only if you feel well enough and can afford it. If you cannot do something she wants because you and your DH are ill, then say so and ignore all her persuasions otherwise.
When your DD gets all wound up wants to have rows with you or do things she wants to do that are too much for you, walk away, quite literally turn your back and walk out. I have a daughter who used to get wound up and want rows. I simply refused to do so, I changed the subject, I made snuffling little non-noises, if all else failed I sat and let her rant without responding or I walked away. It got worse before it got better, but once she realised I wasn't going to have rows with her, she stopped trying. Now she rings me when she is upset, and uses me as a sounding board, but we do not have rows.