They had nothing but time on their hands until they landed on Phu. And during that time, Ben just asked questions about his past, listened to stories from the time they were a family and tried to fit those events into what he thought happened. Sometimes, the pieces fit. Sometimes, not so well.

A few times, when the concentrating look on his face grew very intense, he might even have been using a Jedi mind trick on himself. Something to help him remember who he actually was, and try (not very successfully) to put his torn mind back together.

He was far better at tearing things apart than putting them back together.