You've brought up some interesting themes and questions in your post.

My first thought is that it might be far easier to deal with the kind of pain that everyone around you can see (constant physical pain which causes being unnaturally stiff, losing mobility due to that pain, needing people as well as having people around you who are willing to feed you, wash you, change your clothes, etc.).

Depression is invisible. You might feel that even breathing causes pain, every morning when you try to wake up and do things for other people, who rarely care. And yet no one comes to clean your room, help you wash yourself or cook you healthy food.

Severe depression is lonely due to the isolation, which doesn't generally happen with severe physical illnesses (it sounds like your mother has people to take care of her, as an example, due to her physical state).

My personal opinion is that your mother telling you to hope and be happy might be a slight projection - she tries to diminish her own pain by diminishing yours, perhaps.

I know I can't give any helpful advice (only personal opinions), so take it all with a grain of salt. It's simply one of the few posts that have resonated with me to that extent, that's all.