I don't usually comment, but I had a talk with a coworker today who felt exactly the same and had a similar background. I told her that just by being humble to recognise that she has much to learn is a great sign she is willing to learn and to improve. Most mediocre professionals I dealt with would never be able to question their skills and are unwilling to improve and also they don't actually like their job. The feeling I get from your post is that you like what you do and that is also a major plus. Don't be so hard on you. Things will get better with time, I know they have been with me