This one is a bit generalized, and I might not cover your exact point. But you might find it helpfull nonetheless :)
Creating a base
Humans–although diverse–aren't that diverse. There is a level of similarity between individuals who have a concrete thing or an abstract thing in common. A similar behavior (not completely similar) can be expected from two girls or two men of the same age or two kids who grew up in the same locality. An idea I feel would help would be as follows:-
visualising a pyramid
A pyramid can start from a small detail that's almost common to a network of traits easily distinguishable from other people's .
experience-independent levels
You can start out with levels (traits etc) that are independent of experiences or personality. Like the things you probably fill in boring formal documents. These would include things like gender, age, education, profession, married or not, address , daily routine (it can be independent, maybe) etc. You can go through formal/ semi-fromal documents the get more ideas.
early experience levels
Childhood affects one's entire life. You can now include early experiences that happened in your character's childhood. No special events would mean a normal level that would be similar to many other normal character's. Perhaps the charcters is an orphan and has seen the bad side of the world a bit too early, or a character would be a pushover due to an intimidating older sibling, etc. Your character might not even remember these experiences, but they have a huge impact on their lives as the shape the foundation of their personalty.
personality level
Personality in this case would not just mean ones mannerisms or behaviours, but their reactions, tendancies and many other things. In short- this one is important.
This level would have traits that they inherited from their family (parents, siblings, cousins, gaurdian) genetically or by observation. Traits, mannerisms, belief system. Everything. Almost.
A prince would behave older than his age, the politician's child might be a spoilt brat or even a political mastermind, and there are many possibilities.
experience level
This is where your question comes in.
Now if any kind of trauma had occured after the early years of a person, like the one you mentioned, then it would have a huge impact on every other level. Their belief system is crushed, they are forced to change their behaviour, their whiter personality is changed etc.
Usually this level is quite difficult to write. And yep, it would be quite awkward to ask anyone to share their experiences with us. So the best option would be to
- research {but please be carefull, it's not always the best for you to go so deep in things like these. They sometimes mess with your mind}
- observe people around you, in movies cinemas. If you need to write about some kind of experience, you can read a few pages of a autobiography/book on someone who has what you need. You can watch a movie to get a better understanding visually {I would do that in dire situations}
- try many other methods specialized it experiencing something others did.
You can give it a quick Google search.
Hope it helped, and I would love to read your book when it is finished! I'll try to elaborate on the last point a bit later on