How can I cope with a physically disabled grandmother?

I am the 20 year old granddaughter of a physically disabled 80 year old who is nearly paralyzed and confined to a wheelchair. I sadly lost my other grandmother last March so I’m now forced to rely on her. It’s very horrible to see her suffer like this (she was diagnosed with severe Myositis in 2012 and I think it’s getting worse for her every year) and she can no longer do the things she used to do (like go to Disney or the beach or even stay in a local casino hotel because she likes gambling). She has someone from an in home care company come to help her get out of bed and get dressed every morning (I don’t live with my grandparents but I stay at their house almost every afternoon) and she seems to appreciate that but I think it’s getting very hard for her and it’s heartbreaking to see her like peeing in a jar (her legs won’t open far enough to sit on the toilet) and feeling guilty because she’s living with this condition. How can I cope??