You’ve been dealing with your mom’s depression since you were tiny. It makes sense to me that you feel quite stuck. You want your mom to be happy, but it is at the expense of your own happiness. You don’t want to be unhappy but making any positive move toward your own happiness creates tension and misery with your mom. Yikes.
I don’t think it’s likely that your mother is going to change on her own. She’s been depressed for 20 years. There is no indication in your letter that she is doing anything to help herself besides controlling you. It’s important to remain compassionate toward her but at the same time to work on getting yourself unstuck.
I don’t think you can handle this on your own. You need the practical help and emotional support of another adult. You didn’t mention a dad, so I’m guessing he’s out of the picture. But do you have any relatives or adult family friends you can confide in? If so, ask them to help your mom so you don’t feel so alone and responsible. Perhaps they can help her get treatment for the depression that is ruining her life. Might it be possible for you to go to live with a relative for awhile to give both you and your mom a break?
If your school has counselors, I encourage you to talk to them as well. They may know of other options for you.
This is your last year of school. Start thinking now about what you want to do next. Perhaps start a part time job — both to get some experience and to get out of the house more often. Start to build a life for yourself that is independent of your mother. Think about what steps you would need to take to be able to leave home to live on your own or with friends.
I wish I had a magic wand to make this situation better. Since I don’t, it’s up to you to actively find the help you need either within your extended family, at school or on your own.
I do wish you well.