Koen has provided some very solid advice (as usual).

I have a degree in Physical Education and have never taken a computer science class, although I have done some of the MS specific training. I got into IT because a friend of mine hired me as a programmer and he and the other programmer on staff mentored me in SQL, .NET, OOP, etc... It turned out I had an aptitude for SQL and I've stuck with it for about 14 years now. During that time I've often felt less qualified than people I've worked with, but, as you have mentioned, I always got god reviews and have gotten the job done. That's what I rely on now, even though I often feel inadequate, I get the job done, and I think sometimes that nervous or anxious feeling has helped me because it drives me to learn more. If you can find a mentor that would be a great thing, I've had the person who hired me, and then people like Andy Warren (one of the founders of this site), Steve Jones, and Andy Leonard, and others, that I've met in the SQL community that have helped me along the way. You really should plug into a SQL User group, if there is one near you, because the active people in that group will be willing to mentor you, and in many cases will re-assure you that you are more capable than you seem to think. That's how I get through the tough times.