You've been given a lot of very good advice here, so I won't repeat it all, but I do totally understand how you must feel. I'm 61 and have two GC, aged 5 & 2, who I look after occasionally, and do the school run one day a week. I'm always on my knees with exhaustion by the time they go home! I've had them here overnight on rare occasions, but they are not great sleepers, and both always wake ridiculously early with the birds. I've never been an early riser, so it absolutely kills me!
The overnighters actually all came about when my DiLs mother decided her DD was exhausted from looking after 2 young children, so suggested that we sometimes helped out by having the kids overnight on a Fri/Sat, so they could have a rest/go out for the evening and have a lie-in! Fool that I am, I agreed to it! She's the type of person who's in bed by 9pm and rises at 5.30am, so I guess she can handle it better than me ... she's welcome to it! I'm afraid that overnighters are not for me ... maybe when they are older.
DS & DiL do have a habit of asking me to babysit whilst they go to do some shopping, or something they need to do, but they have been known to be gone for most of the day, enjoying lunch out, etc., whilst I'm running around like a headless chicken!
They know that I'm always available should they need me in an emergency, and when there are teacher training days, and no school. But, as much as I love and cherish my GC, and the time I spend with them, I've got to the point in life now where I feel it's '*my time*'. I'll always do whatever I can to help my AC out with my GC, but not to the detriment of my own well-being and social life.
Taking care of children is definitely a young persons game, and it's a lot harder in your 50s/60s than it is in your 20s/30s. You need to make your DD aware of this, and tell her how exhausting you're finding it all. As well as the fact that you have very little time to yourself, or indeed for your own social life with your partner, which you absolutely deserve.