Question: What decides your personality? Is it just your life experiences and upbringing, or something else?
Answer: There is a famous article (The Freedom) by Rav Yehuda Ashlag – a great empirical, natural scientist of our times – describing the 4 factors influencing, composing who we are.
These 4 factors are our genetic inheritance, our upbringing, the education we received and finally the environment we live in.
It is easy to see from this that after we went through childhood our personality will be shaped, changed, upgraded by the environment we choose for ourselves.
In fact the only free choice we have in life is choosing the environment, since it will supply our value system, our goals, purposes, and it provides the tools and support (if any) in order to reach those goals, purpose.
There is a reason why they say that we are the product of our environment.