Don’t get hung up on writing character backstories. What’s important is knowing how your character will react to situations right now.
We all react differently to trauma and events. For example, someone who grows up in an abusive household may become an abuser themselves, become a crusader for the abused, or just carry on unaffected and lead a normal life.
Backstories are therefore irrelevant for the most part. What matters in your screenplay is how your characters react to things at this moment in their lives.
The only time a backstory will be important is when an aspect of your character’s past will be brought up, or depicted, in the movie — and will have a direct bearing on the plot or another character.
A Simple Rule
If you know what your character would say and do in any situation, you’re ready to start writing the dialogue and actions for your character.