Exerting excessive self control can deplete your mental resources. It can make you give up quicker and more easily than you'd like.
Remember, the amount of self control you have is limited. You can override only a small number of urges at the same time. Tasks like dieting and weight loss that require lots of self control are more affected by depletion than tasks that require less self control. Depleted self control must be replenished before it's available again.
If self control operates like your muscles do, then exercising it can increase its strength - but only if you do it long-term. Frequent exercise of self control followed by rest and replenishment leads to a gradual increase in the strength of your self control. With this kind of regimen you will find that your self control improves even after it fails you on one or two or more tasks. Your self control improves because exerting self control has become more important to you than the outcome of any one task.