Before anything, what is Psychology?
Our usual thoughts are, anything in connection with the mind. Psychology people know how to "read minds". We even think of those in the mental hospital. I don't know if that's how your first thought was, but when I was a kid that's what I think when I hear "psychology".

Well actually, those impressions are partially correct.
Psychology is the applied discipline that involves the scientific study of mental functions and behaviors.

The history of psychology dates back to the ancient civilizations of Egypt, Greece, China, India, and Persia.
Even some half million years ago, people have assumed that psychological problems where caused by evil spirits. To cast these spirits away, ancient healers perform an early form of surgery called trephining. Trephining is making a hole out of a living person's skull with a use of crude stone instruments. Gruesome.

The Different Theories of Human Development

Psychosexual Development

According to Sigmund Fraud, there are different stages of Psycho-sexual development that have strong pleasure-giving qualities at particular stages of development.

Psychosocial Stage

Popularized by Erik Erikson, the Psychosocial stage is where each stage deals with a conflict - turning point.

Cognitive Stages of Development

The Swiss philosopher and psychologist, Jean Piaget, introduced the Cognitive Stages of Development.

This theory is focused on understanding how children acquire knowledge and understanding the nature of intelligence. 

Stages of Moral Development

Lawerence Kohlberg, an American psychologist, well-known for his research into the developing stages of moral development, modified and expanded upon Jean Piaget's work to form a theory that explained the development of moral reasoning.

My Personal Experiences Related to any of the Theories

The most relatable theory for me is Erik Erikson's Psychosocial Stage. I remember when I was 5, how I am determined to do something and when I did it, there's a sense of pride as a child that I did it and I show it to everyone and they're so happy for my little success. But when I did something wrong, I feel like the world is mad at me. (Initiative vs. Guilt)

During my school age, that was the only time I learned to socialize and be real nice to other kids. In class, I always recite and my teacher would commend me because I got a good answer. But at times I say something wrong and the whole class would laugh at me, I feel bad and really ashamed. (Industry vs. Inferiority)

Arriving at the adolescent stage, as a teen, I was always aware of how I am inside and outside. Of how I speak, how I move, how I look, how people see me. I wanted to become something to be admired because I am a young achiever. But being someone admired, I was scared of failure, because that's how I think seven years ago when I was in first year high school. So, whenever I experience negative comments about what I did, I lack so much confidence and feel degraded and all. (Identity vs. Role Confusion)

But as a young adult, I have gone mature. I begin to master life and its challenges and how to deal with it. And the good thing is, I now don't do things alone because I am doing it with someone else who loves me, protects me, and supports me in everything. I also have known all my true friends who were there for me no matter what after years. (Intimacy vs. Isolation)