Of course, I think everyone does this, no matter what position you're in. Liszt often thought about quitting everything in his teenage years, and joining the church. Although the joining the church thing may have been publicity stunts, he really did often think about where his performance life would take him. I have often over the last year, thought about whether or not this whole piano thing would work out. Especially on crappy days where I play horribly. But I look back a few months and look at what I have actually accomplished in such a short amount of time. I couldn't play Mozart's Rondo Alla Turca correctly in January, but now I can play All sorts of advanced pieces. When I look at my progress, and listen to my teachers praising, it makes me fill al fuzzy inside and makes me think more and more that this is what I was meant to do. However I do think that many artists have the 'big' debate with themselves at one point. They break down and consider quitting it all. Whatever you do decide, it has to be firm, straight and sure, or else it will never be as good as you thought it would. If you're always iffy about it, you'll never be happy and you probably wont get anywhere with it.