You sound good, but like you are getting more worn down...
Communities are created precisely because we need to lean on each other sometimes. What communities can you tap for help with your mother? You might want to check your area for support groups also -- support groups for kids caring for elderly parents, support groups for caregivers of chronically ill people, and so on. Yes, these groups also take your time, but I guess it would probably be time very well spent... Worth a try anyway, and maybe you'd find people to share adultsitting duties with... Also, if your Mom is in decline, are you sure she wouldn't qualify for more in-home support from whatever insurance she has? What about her friends who are in better shape, can they visit?
