Just as an informal, nonscientific observation, most people's personalities don't seem to change very much during their lives. There are exceptions in the case of people who go through hugely traumatic events or suffer from brain injury or disease. Some would argue that religious conversions can have deep personality-altering effects. But these are all exceptions to the rule.
I went to school with butterflies of fear every day for years - from primary school onwards - not just worried about being bullied by classmates, but by teachers.