You must be terribly worried. You’ve been watching your mom fade away and have felt helpless to help. She does sound dangerously depressed. I don’t know if her medications are the cause or if she’s truly in such pain that she can’t manage the demands of regular life. But she certainly does need some help.
I can think of a few things you can do, if you haven’t already.
First, insist that she make an appointment with her doctor and get her to let you go along. She can’t see herself from the outside as you can. Tell her doctor what you’ve told me. It may be that there are alternative medications that can give her relief from pain without making her so dysfunctional.
You said that you don’t have a dad but you didn’t mention if you have other relatives to turn to. If other people in the extended family are in a position to help but haven’t known what is going on, it’s time to contact them. I don’t know if she’s addicted or if her depression has so taken over that she hasn’t felt worth any help her family could give her. Either way, “protecting” your mother by not telling other relatives contributes to her isolation.
Finally, you can go to a therapist for yourself. A therapist can help you figure out what you need to do to both take better care of yourself and possibly be more effective with your mom. A therapist can also help you identify local resources. At some point, you and your therapist may decide to invite your mother in to share some of your sessions. Sometimes someone who won’t go to therapy for herself will go if it’s framed as a help for someone they love. The goal could be to find ways for you and your mom to be a team to take care of her and to make sure that essential tasks for running the household get done.
This is a lot for someone who is only 21 to handle. The situation requires that you be more mature and organized than your mother is. I’m sorry you find yourself in this position. But your letter suggests to me that you see things clearly and you are anxious to help. You made an important first step in writing. I hope this response gives you some direction for what you can do next.
I wish you well.