I would like to work part-time but fear mother will not be able to take care of herself while I am gone. Anyone else in this situation?

My mother is 88 years old and I am her only caregiver. My brother and his wife are in denial and only call when it is convenient for them, which is once every 3 or 4 months. I am very depressed and feel so isolated. Am 62 and feel that each year I am not in the job market, it will be that much tougher when I am older to find a job. Further, my mother can't take care of herself and I am her maid, nurse, housekeeper and chef. I am so exhausted and too tired to try to seek social connections. This site is the only means of venting my feelings. I can't hire anyone to come in and help as my mother's pension is small and she could not afford to pay someone $15 or $20 dollars an hour. I feel like just leaving her alone for a few hours a week to do something enjoyable for myself. I do cry daily and pray for some relief. My son and his girlfriend and baby live here as well and I also help care for my grandchild. Between taking care of mother and the baby I feel like running away at times and dream of having no responsibility except to take care of my own self. I know this sounds somewhat selfish and that I should be thankful to care for my mother as she sacrificed for me while I was growing up. I am reminded of this almost daily and feel guilty for wanting my life back. I don't know what to do but I know I would love to work part-time to earn some income and be able to not depend on anyone when it comes time for me to be on my own. Thank you for any advice you can offer me and God Bless you for caring.