I have a mental health question - how is personality determined?

That's a very broad: question for 400 characters. In brief, personality develops during early childhood mainly before the age of 5 or 6 years. It continues to develop throughout childhood, but the base is built early in life. That is why much of analytic psychotherapy focuses on early relationships with parents during the formative years. It is also why teaching parents about child rearing is important.

Its genetic guys: Genes play a greater role in forming character traits -- such as self-control, decision making or sociability -- than was previously thought, new research suggests. A study of more than 800 sets of twins found that genetics were more influential in shaping key traits than a person's home environment and surroundings.