Changing your personality is completely possible. Until about 10 years ago it was generally accepted that what Bruce said is right, that after your late teens / early 20s (depending on who you asked) the brain was basically set and it couldn't be changed.
About 10 years ago the scientific community, due to an overwhelming amount of compelling evidence, changed it's collective mind and it's now generally accepted that the brain can be changed right up until the day you die.
For anyone interested in looking in to this further, I wrote about the link between neuroplasticity and personal growth
here, and if you're dead set against reading my blog, the process by which the brain changes itself is called neuroplasticity or brain plasticity.
So to answer the OP's question, personality change happens by way of neuroplasticity, and neuroplasticity takes consistent, patient, hard work.