You can change your personality. I changed from a cynical, depressed pessimist into a happy, hopeful realist. I was painfully shy and very insecure; now I am a friendly introvert—I still get overwhelmed in crowds, but I feel much less intimidated in social situations.
For me, it involved observing people whom I admired, and emulating those character traits that I wanted to incorporate into who I am. It took time and conscious effort when making everyday decisions, and learning to not reflexively react emotionally, but to pause and think and choose my actions based on how the person I want to become would handle that scenario.
At first it felt unnatural, but in time and with practice, those attitudes and behaviors became a part of me and I transformed into the person I always knew lived inside of me. And my scores on those personality tests have changed in the past 20 years.