Can we change our personalities? In psychology, a difference is made between personality attributes as a state or as a trait. For example, a usually outgoing and sociable person might find himself uncommonly shy in the presence of a woman he finds attractive. He is not shy "by nature", but only in this situation – his momentary shyness is a state. Another person, on the other hand, might be generally timid around even close friends and family members – his shyness is a trait.

Can this person, who is always and basically shy, decide to change himself and not only learn to overcome his fear, but really and truly become extroverted from the ground up?

Shyness is just an example here, I'm not interested in therapies for shyness, but in basic deliberate personality changes that take place within a reasonable period of time (a few years, not half a lifetime).
This is my opinion, based on what I know about how the brain works. As all process within the brain are physiological processes. It must be acknowledged, that what we know now about the brain, will, most likely, seem simplistic, to future generations; as more research is done. Will provide some quotes from the links I've provided, as I have time.



I don't wish to get bogged down in a debate about what “personality” is, traits, state etc. For the sake of this argument, let's assume personality is the most "natural" way of being of a person. Those innate impulses that arise from the "subconscious" parts of the brain, and these can either be allowed to flow (depending on social context and impulses) or require vigorous conscious action by the frontal lobe, to inhibit action upon such impulses. This latter comment regarding inhibiting these does not pertain to the actual “personality”, but to the efforts required to make conscious change and is a measure of the difficulty in bringing about fundamental personality change.

So for the purpose of this argument, I am viewing personality as the subconscious impulses that arise from the brain.

Which then brings us to the question. What is causing these impulses? To make it short, Long Term Potentiation) LTP of various neural pathways within the brain.  There is no guarantee that a connection between two neurons will fire. It is the priming of the synaptic cleft , the membrane endings and contents of both neurons that increases the likelihood that the a connection will form between two neurons. When a pathway is formed, that is “permanent” (as close to permanent as something can be in the brain), it means that pathway of the brain has been Long Term Potentiated. So the neurons fire to form that thought/action/feeling, effortlessly and reliably.

Which brings us to the next question. How is the brain Long Term Potentiated? To make it short (again), for “ordinary” activities; like learning the piano, or not to interrupt people when they are talking; reiteration of experience. The more dull the consequences of each iteration, the more iterations required.  Which works, conversely, as in the case of trauma, where there is a cascade of chemicals throughout the body that assists in the LTP of the neural paths being used during the experience. 

This then brings us to the next question. How does change one's personality or one “undo” or “reduce” LTP of parts of the brain, we’d like to change and bring about “LTP” of pathways in the brain we find desirable. Now this is the key to answering the OP’s question, Can personalities change within a period of a few years (as opposed to decades). 

As already mentioned in one answer, well-practiced psychotherapy (whatever you’d like to call it) relies on the concept of undoing LTP and LTP different pathways. Also, as mentioned in another answer, trauma creates a rapid change in LTP of pathways in the brain.

Now, I say this, using the logic of my above points; From my observation of human beings, my own efforts in life to change my personality and study of neuroscience and interest in psychology and psychiatry. I can honestly say, I have come to the conclusion, that it is most difficult and unlikely that any significant change can be made to someone’s personality, without the use of severe methods. One can practice overcoming (in this case), the action of behaving as a shy person, but the experience of social anxiety will remain and lag. It will eventually change. As the brain is plastic and we can mold it through repetition. However it, usually, would take longer than a few years for the internal changes to follow. This is something that can be achieved over half a life time, but the outward change of behavior, does not equal an inward change of personality; it can be a reflection of a person’s willingness to bring about change (and is most certainly a step in the right direction in achieving change). Look at many disciplines, martial arts, many spiritual vocations, it takes many years for the person to evolve. Which is a change in personality. And in these cases this is people living a full-time existence to transform their personalities. So the poor, every day, folk, with all of life’s pressures bearing down upon them, it makes it even harder to consciously override our natural impulses. 

I should add, that it's the gamut of human experience that shapes who we are, combined with the natural chemistry. I have not covered this as in depth as would be to give this discussion full due.

As this site is a minefield of potential controversy and arguments over semantics. For the purposes of this answer I am using “words” that I am defining for the use of this passage here, and the debatable semantics of their use is outside of the scope of the OP’s question. So please comment on the merits and principle of my answer, that addresses the question and not points off topic.



BEHAVIORAL AND BRAIN SCIENCES (1997) 20, 597–655
Long-term potentiation: What’s
learning got to do with it?
Tracey J. Shors
Louis D. Matzel
http://hargreaves.swong.webfactional.com/bbs97.pdf