There is some truth in what you say, we act according to our own values,beliefs and previous experiences and we tend to justify or accept our behaviour afterwards... so we remain true to type.. However, we cannot deny that sometimes things happen that change us, make us re-evaluate ourselves and our behaviours.
I don't know how someone would turn into anyone other than their own self. Unless, of course, they are acting or impersonating someone else.
But if you mean no matter your environment, you would be the same personality ... no. It would be almost impossible to prove, without the ability to time travel. However, there are cases of siblings separated at birth - raised in different environments, with different experiences - who turn out with vastly different personalities.
