deep down

「深い」という意味で知られるdeepですが，deep downでif you know or feel something deep down, you secretly know or feel it even though you do not admit it（LDOCE）という意味があります。『プログレッシブ英和中辞典』（小学館）では「心の底では, 本心では」と説明しています。今回の記事でも「心の底では間違っていると分かっていたが」と解釈できます。（Koyamamoto）

Avoiding Cold Feet Down the Aisle
His charisma was big enough to make his bad habits seem small, more like quirks than flaws. The cigarettes on his breath; the extra weight around the middle; the indifference to clothing and appearances — surely these were minor things, correctable in time.
In the months leading up to the wedding, in 1988, even the fact that he’d been living with his mother at age 38 seemed somehow explainable, if not ideal.
“How about that for a red flag?” said Jincey Huck, a state court employee in St. Louis, of her first husband, who has since died. “Deep down I knew it was a mistake, but I wanted to be married, I wanted kids, all that. I had cold feet the entire time,” said Ms. Huck, now 51.
http://well.blogs.nytimes.com/2013/01/21/avoiding-cold-feet-down-the-aisle/?hpw