You must establish a firm psychological stance early in your graduate career to keep from being buffeted by the many demands that will be made on your time. If you don't watch out, the pressures of course work, teaching, language requirements and who knows what else will push you around like a large, docile molecule in Brownian motion.
That quote is from an excellent list of tips for graduate students, entitled "Some Modest Advice for Graduate Students," by Yale professor Stephen C. Stearns. I particularly like the section called "Psychological Problems are the Biggest Barrier." As a psychologist, I see all the time that the grad students I coach are plenty smart enough to do the job. What impedes their progress are the psychological mine fields that are in their path. I suggest reading this article if you have been running into these mines.