The fundamental attribution error is assuming that the actions of others are caused by internal factors- things you have done to/for them, for instance- instead of external factors to you- their relationships, challenges, and lives in general.
This is a piece of advice my Mom always gave me when I was younger. She told me that just because someone treats you badly, it isn't your fault. It's important to consider the other factors.
There is a similar concept advanced by young adult author John Green, especially in his book Paper Towns. He terms it "imagining people complexly," looking at other people in the context of their own lives, and respecting that they are also immeasurably complex, and cannot be summed up by quick titles, labels, or stereotypes.