Try not too look at design as just a serious of skills acquisitions. It can teach you a huge amount about life and yourself. You'll learn that good design takes intellect, wit and a huge amount of labour.
If you truly believe that you can be brilliant then go ahead and do it to the best of your ability. Your years and your passion are an advantage.
As for the questions about education - I'd go for nothing less than a Bachelor of Arts in Graphic Design - if you cannot get onto a good course find out why, improve your ammunition, and apply again. Marketing will not teach you how to produce great design.