Well I am in HMPOR withdrawal so I will post an idea that I have about the origin of the "unverbalizable fear" that Harry has while under the sorting hat about going to Hufflepuff where he will be happy. This idea is based on the few descriptions of the relationship between harry and his father in the early chapters

I was going to post the excerpts in this comment, but it is quite a bit of content so I will abstain for now. If someone wants me to post the excerpts I will. For now suffice it to say that Harry does not feel respected by his father and the only positive feeling that his father is said to display towards Harry is pride.

His eyes glanced over to his father Michael Verres-Evans, who was looking stereotypically stern-but-proud

You might say that his father dropping everything for a last minute book buying spree was a very kind thing to do. Indeed Harry himself says that his dad is "awesome" because he buys him books, and uses the memory later when trying to cast the patronius charm. However, considering the lack of respect and affection that Harry's father shows I have to ask if the book buying spree was really for Harry. It seems more likely to be an attempt to help Harry "be all that he can be" not for Harry, but for himself. So he can continue to feel proud about the achievements of his child.

Children need the expression of love and affection from their parents in order to feel secure. If all Harry can get from his father is pride then it will be very important for him to maintain it. I submit that Harry's unverbalizable fear is an unconscious fear of abandonment(death for most of human history) if he loses his fathers pride by choosing the path of happiness over achievement.