The Salt star, who serves as a Goodwill Ambassador for the organisation, struggled to cope when she met people living in extreme poverty, famine and war.
She told Total Film: "My first trip changed me completely. When I first travelled I was just a ridiculous crying mess.
"These people have had their families killed, had their houses burned down, become stateless. They're not crying because they don't have time for it and I'm not helping them by just feeling for them. I had to stay practical and focused."
The actress was advised to write her emotions down in a diary to help her process the dreadful things she had seen and Jolie admits the experiences changed her as a person.
She added: "It's funny because you are always asked to sum up who you are and your personality and your changes and I've never had therapy so I don't know exactly how I did that (open up in the diary).
"I just know that I opened myself up to want to learn more about the world and about other people and things that are going on. So I changed."