Writing My Portfolio

As I write my story for my portfolio, I've found myself encountering a major problem. Since roughly half of the story is set within a diary, I am finding it difficult to keep in character while at the same time providing the needed exposition for the story. I'm trying to incorporate a fabricated mythology in a 1950s veteran. It's proving to be a challenge, as the words I want to use to describe the mythology aren't necessarily the words that my character would use. I'm having trouble finding the balance between my exposition and my character. On the one hand, I could describe a whole parallel universe regarding this fabricated mythology. However, this would either seem boring/uncharacteristic of my character. On the other hand, I could be quite ambiguous as possible and leave the reader to fill in the details, which isn't very fun for neither the reader nor me. I think I'm going to have to look at some writing resources online to help me better understand that balance.