What can I do with a person with advanced dementia?

I've been my mother's family caregiver for seven years and have watched her go from moderate cognitive impairment to very advanced dementia. She's lived in three different facilities and I go to see her once or twice a week. I've also handled all her bills, medical appointments, taxes, ordering medications, buying diapers, everything. My brothers live across the country and usually visit once a year for a couple of days. I'm spent. The visits with my mom have gotten more and more difficult. She usually recognizes me but she's barely a shell of the vivacious and intelligent woman she used to be. She can't put more than a few words together and sometimes she just moans, like she's aching to get out of this life. I get so depressed when I see her. I try to hold her (she's very affectionate), play music she likes, take her outside, bring her treats to eat. What else can I do? And how can I keep from getting bummed out every time I see her?
(As I look through the topics it's hard to know whether to list this under Caregiver Burnout, Depression, Activities & Recreation or Life as a Caregiver! I'm dealing with all those things!)