It sounds to me like she very well may be at a level of disability to qualify for nursing home placement. If not, then she must have some self-care capacity that should let you have some more time to yourself. Possibly her depression is the most disabling condition that she has.

I'm glad you have a fiance. Do you plan to continue living with your mom after you get married? You might want to start looking into how you could arrange to not have that happen. If your mother is mentally competent ( even if she is very depressed,) it does not have to be your responsibility to "put" her in a nursing home, or wherever. She needs to decide how to arrange for her own needs. She will not bother doing that, if she totally doesn't have to because you have covered all the bases.

You've been doing this for too long. You can say "enough." It will probably require you to live elsewhere to escape being overly depended upon. That's hard to do when you haven't been able to focus much on your own development with respect to education and employment. Now is the time to start. You are still young, but you won't always be. If marriage and moving out are not on the horizon yet for real soon, then do something else to distance yourself. Like, get into a vocational training program. Let your mom know that she will have to arrange how to meet her care needs while you are in school. If she finds that a big imposition, then she doesn't care much about what becomes of you in life. To deserve a daughter's love, a mom needs to have a mother's heart.