I think unlimited makes a good point and it may be that your thinking is holding you back a bit. At this point in time, I don't think you need to be thinking "how can I make a great career for myself?" I think you should be thinking "what is my first step?"
The suggestion to attempt uni (if you decide that is the right thing for you) as a mature student, maybe after an access course, seem sensible to me. But first you want more confidence, more experience of life, and that will come from work.
If you have only recently emerged from MH problems severe enough to interfere with your studies, and have debts, I would ignore all advice about setting up your own company: doesn't sound like you are in the right place for that at the moment. Nor am I so sure about the RAF advice- have a feeling the army look quite carefully at medical history.
I'd try to get work for now, any kind of work. Much easier to apply for a job from a job. Send in applications to any kind of work: retail, catering, whatever you can think of. Masses of applications. Accept that you are still very young and that entry level may be what you have to do. For now.
Then once you are in that job, think about the skills you are accumulating, think about what you are learning about yourself and your strengths. Think about whether your type of work, by then, is something you might want to develop (e.g. become a manager) or whether it helps you to focus on different things you could do.
21 is so young. My own dd is pretty well the same age as you: she struggled with depression in Sixth Form but is now doing a Foundation course and hoping to get into a BA course next year. There will be aspects of it she will struggle with due to physical disability, but she will have to be inventive and work round that.
Sometimes her anxiety has the same effect on her as it seems to have on you and she gets very despondent, but she is getting better and better at recognising when it is depression speaking, and accepting that you can get to the same place by more than one route.