Wounded Hero/ine

Also known as a "hole in the heart" (Michael Hauge), a character's wound often defines their internal conflict. It's that missing piece in their lives that affects them emotionally, and stops them from entering into a loving relationship.

Ex: My mom left us when I was twelve. I'm not worthy of love.
Ex: My dad beat me when I displeased him. I'm not good enough.
Ex: My checkered past has ruined all chances of happiness for the future. I'm not worthy of being trusted.
Ex: As a child, we moved too often for me to make any friends. I can't be one now.
Ex: I was born with [disease/issue] and I don't want to pass that on to my children.

Once the reader understands the internal conflict, don't pound it into their skulls with needless repetition. The reader gets it. Now show how it affects the characters daily life.

Make use of subtext, throwaway lines in which the characters themselves may not even realize how much they've given away.

Hauge also has a phrase known as "the killer share" to describe that scene where the character reveals the wound and opens up enough to explain the hole in the heart and/or how it got there and/or how it makes the character feel.

Internal conflict is not why the person can't fall in love, but why s/he can't act on their feelings. They want the other person, but truly believe they can't have them.