Willpower or desire alone is often not enough or sufficient to change long standing or deeply ingrained habits or behaviors.
All habits, learned behaviors, and attitudes reside within our subconscious mind.  Willpower, along with logic, reasoning, and decision-making, are functions of our conscious mind.  By contrast, willpower, logic, reasoning, and decision-making are not functions of our subconscious mind.  This is at the core of understanding why hypnotherapy is such an effective tool.  In short, hypnotherapy allows one to bypass flawed logic, flawed reasoning, flawed decision-making, and impaired willpower to bring about behavioral change.
If a negative behavior or undesirable habits are deeply rooted in your subconscious mind, or you have exhibited a behavior or undesirable habit for many years, then willpower alone will not be strong enough to change or eliminate the behavior.  This is why people fail to achieve goals, rehabilitation, or resolutions. Willpower or sheer determination can help you to accomplish a goal, but in most cases, willpower alone will not alter long term patterns of behavior. This is because our conscious mind frequently sabotages willpower thereby obstructing change.
There are many reasons why we behave the way we do. Most of us become comfortable in our own skin. Change can be intimidating because it alters the way things have always been. The way things have always been is known as our homeostasis. It is who we know ourselves to be, both strengths and weaknesses. Frequently the conscious mind will sabotage you to keep you where you are, so you remain in your comfort zone – even if your comfort zone includes undesirable qualities.
However, hypnosis, when used by a trained hypnotherapist, can affect change directly within the subconscious mind where all habits, learned behaviors, and attitudes reside. Consequently, hypnotherapy is a very effective and relatively quick and easy path to success.  You may even want to think of post hypnotic suggestions as being analogous to software patches downloaded to a computer program or to the operating system of a computer.