I can't say Biker Scout is wrong...
I don't have a design degree, only a few months at Art Center in California and a state University before dropping out, and I got pretty high up the environmental graphics/graphic design food chain before going on my own.
I have worked around 7 design school instructors by the way, they worked in the field and taught.
My experience is slightly different, all the designers I know personally, went to school and are making decent money. That may be because I have been employed longer than being self employed. Most of the design positions I have ever held, every designer had a degree or were in the process of getting one. I was taught concept design while being taught mechanics by a few mentors and I picked it up very quickly. I am also a huge book and magazine reader/collector. I did not learn it on my own, I was taught on the job.
I have looked at a million resumes and portfolios that came into the design firms I worked at. I would say that 80% were marginal to poor (to me). But of that 80%. More than half went to marginal schools. But the other 20% usually blew my mind.
I have not seen enough of your work or know you enough to make a determination if you need design school or not. I say before you do anything, get some work together and show someone who has no emotional interest in you to give you a fair evaluation of your work and what you need to work on. You might try participating more here showing more of your work for critique. The premium area might be a little safer to show if you are afraid a client will see the work online.
If you intend on picking a school, like any profession, there are the pedigree schools, and then there are the others. I'm not saying you can't learn design at the others or on your own, but there are schools that have a better reputation for getting designers in the workplace and if there is something specialized, like Pixar animation or Disney type signage, it helps to know the schools that can get you the job/work.
Personally, I think it's very difficult to learn this on your own with talent, if you have none or very little, you are going to have a very rough time of it when dealing with higher end clients. If you have lower end clients, it may not be a big deal as most do not know the difference unless you put good design in front of them.. and even then.