If I understand the question correctly, the sudden personality change did happen to me as follows:

Up until about 7th grade, I was somewhat happy and outgoing. I had all the cliche fond memories and experiences of youth until then.

In the summer of 7th grade, my family moved to a more affluent neighborhood [from a poor neighborhood where a cute face got you mild attention]. The problem was I was okay in my poor neighborhood. Once I moved to the new place in 8th grade to 12 grade, I was a 180 degree outcast. The new world valued interesting campfire speeches and family vacation summaries, whereas my family had mental and physical regurgitations of diseases.

I basically became a non-consequential person overnight in my new surrounding, and developing asocial behaviors was my defense over many years. Once college and adulthood came, I realized my asocial defense mechanisms did cost me because I bottled myself up to even the cutest crushes of my life that tried to talk to me.

I think most of us are here dwelling over mis-opportunity, if I am reading this place correctly. We are trying to salvage what we can if we aren't too damaged beyond repair.