Fac, si facis: "Do it, if you're going to do it"
What is the difference between working for a company and owning a company with employees? What is the difference between a snowboarding going to the Olympics and an amateur riding in the terrain park?
When looking at the two people - the professionals vs amateurs - the difference between the two is the amount of work put into the craft. Sitting down and saying "I'm going to be a professional athlete" is not enough. Thinking and accepting this idea that you may have is not enough to get you to that level. You must work.
Along the way though, our ego is going to get in the way. Our ego wants our ideas, wants us to aspire to something, it wants that attention and glory of saying you are going to do something (echoing a previous post, The Dope Addiction). But in saying something, we are not progressing - there is no work being done.
"When you are not practicing, someone out there is, and when you meet him, he will win"
Practice. Work. Improve. Every time you sit down to work on that side project, to study, to prepare for an exam, to do your homework, remind yourself that the work you are doing is an investment in yourself. Do the work.