Tough question for many of us. "I am what I am," some say. "That's what I am and you'll have to take it or leave it." On the other hand, if our genes, life experiences and thoughts have a bearing on who we are, then we also have a say in who we are and who we want to be. We can shape our future. Julian Baggini during a TED Talk clearly expresses this idea. To view his presentation click on the link above.
We can train our brain in many ways. Explore what you want to be, and then begin the process necessary for you to reach your goal. Try to keep your goals in the reasonable range. I will never be able to run a 4 minute mile, for example. On the other hand, you might be able to.