There are many college programs for people with mental illnesses, grants.gov lists grants that are available for students and most just require a little writing. Be sure to get in with the office for students with disabilities to get a college education, and get a great job.
It is also possible to even get loan forgiveness if you aren't making the money from your college degree that you thought you would when you have a disability, and also due to the current administrations policies.
But if you currently can't find a job you enjoy, or your mental illness is tough to deal with, school may be the answer for you, and down the road you may feel the capacity and the experience to work. School prepares you for work, so why not take advantage of it?
A little bit of information for the student on SSI - student loans are not counted as income on SSI so this is a way one can support themselves on SSI while they are looking for a career they can go into, perhaps a career in something they will work at part time. Since disability qualifies you if you have a condition that will last one year or longer, it is technically feasable for you to leave your disabled status if you are a very functional diagnosed individual but I would consider talking to a psychiatrist about it as it is not always easy to get back on SSI a second or third time.