I have never believed I am the opposite sex. I was born female, and I am female today. Whilst I have pharmacologically and surgically modified my sexual characteristics, I know it is impossible to truly change sex. Socially I feel I fit more into a grey area. I experienced being treated as a girl until my teenage years, and then I experienced being treated as a trans boy/man. As an adult I have been treated as other men are, as people are not aware of my biological sex.
In terms of what triggered my choice to transition - I had a deep distress related to my genitals from a young age. My first memories of this are from when I was in nursery. I'm not sure exactly how old I was, but I'd estimate around 3-4 years old, as I went into school reception year not too long after. When I saw myself in the mirror, or had a bath, I felt terrible distress, but I did not know why or what it meant.
This distress got worse the older I got, and I would lie awake at night wishing I had been born with a penis. That feeling has never gone away. I have no idea why I am like this though.
I was very fortunate to have a mother who never really enforced gender stereotypes on me or my brother. We were treated very equally and played and dressed in whatever kind of clothes we were comfortable in. As a young child I wore 'girls' clothes, but they would always be practical tshirts and trousers, as I would always be out playing with other children on the street. From the age of about 10 all my clothes came from the boys section, after I asked my mum in the shop if I could get them. She was always a tomboy growing up, and I don't think she really thought anything of it, other than me wanting more practical and comfortable clothes, which girls clothes are sadly often not.
I had a very accepting friend group in primary school. I enjoyed running, and there was a nice group of us, both boys and girls, who would spend our break time having races. As such, my differences weren't so apparent. My only real distress related to swimming lessons due to the need to undress in an open changing room, and I would always pretend to be ill to try and avoid it.
However, when I started high school I suffered very severe bullying, mainly from other girls. I changed my appearance to try and look more typically female, and tried to push my feelings down so I could just be 'normal'. Unfortunately this resulted in mental health problems, and that combined with continued bullying resulted in a suicide attempt when I was 13. My parents took me out of the school and I went to a new one. Unfortunately, I had similar experiences there. I ultimately told my mother I thought I was transgender. She was really devastated by this, though she never judged me, I would just find her crying when she thought I wasn't looking. My father was not accepting of it for many years until I was an adult.
My parents divorced and I asked my mother if I could change my name to that of a boy, and try and live as one. She agreed to this, but the school was not on board. They compromised by adding trousers to the female uniform, which I wore, but I was not able to socially transition until I was 16 and went to sixth form college. The staff there had never had a trans student, and did not really know how to deal with it, but all were very kind and tried to be supportive however they could. I still did get bullied by some students, but I also found a very loving and supportive group of friends who accepted me.
In terms of who I am as a person, I don't think that has ever really changed, though of course I have grown and matured as we all have.