I have just watched "All Together Now (2020 film)".
And the girl says "it was all on me".
This is the plot that leads her to say that.
The girl named Amber Appleton and her mom Becky are temporarily homeless after Becky leaves her abusive boyfriend Oliver. They sleep in the school bus where Becky is a bus driver.
One day, Becky's employers discover that she has been living in her bus and fire her.
Becky convinces Amber to get back to leave with Oliver, but Amber refuses because she thinks it's not safe to live with Amber. Amber runs away from her mom.
Both Becky and Oliver are alcoholic.
A few days later, police officers go to Amber's school and inform that Oliver and Becky have both been killed in a car accident, presumably while driving drunk.
And the girl says "it was all on me" because she thinks she runs away from her mom when her mom needs her.
In dictionary, "on" means "causing someone problems"
30 CAUSING SOMEBODY PROBLEMS used when something bad happens to you, for example when something you are using suddenly stops working, or someone you have a relationship with suddenly leaves you
Suddenly the telephone went dead on me.
Dorothy’s first husband walked out on her.
But "on" also means "to be responsible for something"
17 — used to indicate the person or thing that is responsible for something
The drinks are on me. [=I will pay for the drinks]
They blamed it on me. [=they said it was my fault]
Some American people say "It was all on me", does "on" mean "I was responsible for it" or "it caused me some problems"?