Changing your personality isn't as simple as thinking, "I will be a better person." The mind is an accumulation of everything experienced and done throughout one's lifetime. You must be consistent and resolute in examining and controlling your thought processes.
What is the Nature of a Strong Willpower?Edit
People with determination do not follow their whims, because they know that these thoughts are fleeting. They concentrate on long-term goals, and evaluate the worth of each action in reaching these goals.