Why is self-control so hard?
If we always "know" and "do" what is best for our well-being, then we wouldn't be talking about self-control or New Year's resolutions. But our brain is evolved over hundreds of millions of years in many different life forms and environments. And hence we carry jungle of many different programs in our head — with their own strengths and weaknesses. One part of us may want something, yet some other part lures us into something else; one part may seek a long-term goal, yet another part may fall for an instant gratification; one part may be interested in something, yet another part may be scared to pursue. One part may feel sad, yet another part may feel angry about feeling sad, and yet another part may feel angry about feeling angry. Our mind is fragmented like this in so may ways.
Different tasks are carried out and managed by different programs in the brain. Depending on the situation, these programs some times cooperate and other times compete. But the brain is evolved such a way that one program cannot "directly" control and change another one. Because such "directness" is too dangerous — such as, simply turning off "sleep" or "pain" at will. We would easily end up destroying ourselves. Nevertheless people still manage to destroy themselves in many other ways such as taking dangerous drugs.
Self-control is hard, because there is no "direct" and easy way to control or change the programs. We have to learn to use "indirect" approaches — such as, using rewards, tricks, bribes, exploitation and threats... convincing with beliefs and logic... arousing greed, hope and fear. In other words, self-control is very much like how we control, influence, exploit other people. As this has become ingrained part of us, we may not aware of it for the most part.
Self-control vs. Self-discipline
Self-control strategies such as will-power usually hard to sustain it in the long-run. Many feel-good tricks and exploitation may seem to work, but people are often stuck in them like addicted to alcohol and drugs with many harmful side-effects. Most of our suffering, conflicts, chaos and wars in the world are rooted in these strategies!
The fundamental problem is that the mind is fragmented — which is, opposite of harmony and peace. The common self-control strategies only further strengthen this fragmentation. As long as we divide things as good/bad, right/wrong, divine/evil, etc., we continue to strengthen it. As long as we try to chase after something (craving/ greed) or run away from others (aversion/ hatred), we continue to strengthen it. As long as we try to control it with brute force and will-power without proper understanding, we continue to strengthen it.
A skillful approach should bring harmony among all its parts and create unity in the mind. It should be a wholesome approach (this is the original root of 'holistic' and 'holy'). Mindfulness is such a self-discipline approach (MindFULness = WHOLEness). Discipline (derived from same root as 'disciple') means willingness to be open and listen, and ready to learn. Here, instead of controlling and suppressing, we learn to relax and ride the wave; instead of allowing external events and situations to crush us, we learn to ride them; instead of mindlessly applying will-power, we learn to listen and apply wisdom-power!
All the best!