A childhood belief about reading

When I was reading a biography as a fourth grade reading assignment, I first encountered in print the word "determined", as in "He was determined" (meaning firmly resolved), and that phrase occurred a lot in the book. Most of the way through the book, I was reading an extra "d" into the word near the end, and thinking I was encountering a brand new word "determinded", which I was thinking of as rhyming with "better minded". So I assumed it meant some state of mind implied by some prefix, "deter-" (rhyming with "better") and previously unfamiliar to me. Since the word occurred frequently in the book, I came to understand it from the context to have pretty much the actual meaning of "determined" in that context. So I assumed the prefix "deter-" meant something like firm and unchangeable, so "determinded" would mean having such a firm and unchangeable stste of mind. So I was pretty much understanding the correct meaning, in spite of reading it into an imaginary word. Near the end of the book, I finally noticed that the word lacked the extra "d" that would have mad it it end in "minded", and pretty soon thereafter, I guess I realized that it was the familiar word "determined". Guess I was a bit disappointed that I hadn't learned and deciphered a new word after all.