I am a mathematician and occasionally a good poker player. I have read countless articles and books, have several years of playing experience, and do my own research with sims and databases. However, while I think I have a good theoretical understanding of poker, the hardest thing to control is emotion. The winning player doesn't tilt, plain and simple. I can play for several weeks and amass a large bankroll, but (especially in NL) it only takes a single moment of irrational, poor judgment to lose it all.