Yeah... I guess that was a bit over-arsehole of me, I came off too strong about it, it's just the if part of the title, I still think that if you truly want to you can change yourself by deciding to do things differently, say if you have been a complete slob all your life, then suddenly you reflect on yourself, get your act together and clean up, get a good job, and so on, would you not have changed your personality?
The key to changing your personality is to reflect, reflect on what you did during the day and think of how you may change it, then try to implement these changes in your life and slowly your personality will change.