Rob
I agree with RAFE on the back up plan. When I was your age, in fact younger, my heart was set on becoming a professional footballer and I was going to be nothing else. After trials and spells in a pro club's youth team in London where I thought I was developing well I was "let go", several trials later it became clear that I wasn't going to make it as a pro and I suddenly found myself having to think about the realities of getting a job and I was totally unprepared. I hadn't a clue what I wanted to do, my education had suffered because of the vast amount of time I had spent at training grounds instead of studying. Coupled with my extreme disappointment at being told I was not going to make it in the career that I had my heart set on left me devastated and directionless. That's a bit different to your situation because you have many years in which to make it as a pilot, unlike most wanabee footballers, but the point is if you make a back up plan, you can buy yourself time while still pursuing your dream and still doing something you enjoy. Oh, and don't forget, have lots of fun along the way.
Cheers
Sean