Hi! I can really relate to your feelings of obligation. I feel the same with my alcoholic dad and I am also an only child. He is also alone.
I think these feelings are part of unhealthy attachment patterns in childhood too where we felt responsible for the parent as opposed to feeling the parent was responsible for us.
She is lucky that your are even there and supporting her. You don’t have to be even though she is your mother. You are choosing to, and your actions are honorable yet it would be okay too if you felt you couldn’t given who she is and what you have experienced.