Why is a person who has schizoaffective disorder Bipolar type I considered mentally disabled?

Why is a person who has schizoaffective disorder Bipolar type I considered to be mentally disabled? I always thought being mentally disabled means that the person can’t be independent but I think I can if I can get 30% assistance.
I have this mental illness but I am stable with medications. My counselor told me I can’t work because stress makes my illness worse.
Does this mean I have to depend on social security disability for the rest of my life?

I always thought I would be able to live in a big house and have a nice car and take vacations around the world once a year.