One of the many gems of advice that famed investor Warren Buffet gives, is that to be successful you should operate within your 'circle of competence.'
That is to say, you have to figure out where your aptitudes are, where you have an edge, and play within it.
If you play where others have an edge, you're going to lose.
Over time, work to expand your circle, but never fool yourself about where it stands today. Don't be afraid to say "I don't know."