I too am a foster parents to my grandson, he has autism and health issues, he has been with us for 10 years now, came to us at 3, we made the decision between us to foster him, although I'm down as main carer, he goes everywhere with us but that was our decision made together, I don't know your grandchild age but there is only so much mum or foster mum can do, I don't sign my lad up for anything that puts pressure on us, driving is tiring and so I cut down, we shared the care until recently when my husband had a stroke, the social services will give me help if I need it and ask for it, we often have our shopping delivered, after I drop him at school I take time out for me, I get a coffee and sit for a while,
I need this space, I wonder why there is separation issues? Maybe find out the reason, my husband and myself have had separate bedrooms now for 3 years, it works for us, though I would never have my grandson in my bedroom I would leave the door open and perhaps have a baby monitor in his room, he has a night light, when he was younger,
Really you need more support, perhaps while you are on your own go for a coffee, or walk, if he dosnt want to go with you then go yourself