Why can't I emotionally & mentally disconnect when my husband is sleeping or watching TV?

I hope I can articulate what I'm trying to convey. Quick rundown: I live in Texas; I'm caring for my husband (Bill) who is 72; I'm 63; Bill was diagnosed with dementia in June 2010 and has just entered Stage 6; I work full-time to support us, but have been blessed to be able to work from home 2 days/week; Bill goes to dementia day care the other 3 days while I'm at work because he can't be left alone and needs help with most things; I'd like to keep him home with me as long as I possibly can.

OK, so here's what I'm struggling with. When I'm with Bill in the evenings, when I'm working from home and on the weekends, there will be times when he sleeps or watches TV in our master bedroom (our master is his 'world' now and he feels very secure there.) Anyway, after his needs are taken care of and he's sleeping or watching TV, I still can't just relax. I can't seem to disconnect mentally or emotionally from being overwhelmed with caring for him - even when I know he's O.K. for a while. It would be a perfect time for me to move on to personal interests (not just housework, cooking, etc.) I can do it to some degree when I'm working at my job because I have so much to do. I think I feel so vested in his care that it's difficult for me to let go for a while. I've often wondered if someone came over to be with Bill so I could go do something for myself, if I would even be able to enjoy myself and temporarily disconnect my thoughts and emotions from his care. As I read this, it almost sounds like a control thing. I'm not really sure, but I was wondering if anyone else can relate? I think this is one of the reasons caregivers burn out - it's not only the physical work that wears us out, but the emotional and mental investment as well.