We all have, if you've ever felt so crushing a loss.

felt后不是宾语从句，而是名词词组 so crushing a loss.

通常当一个名词前既有限定词，又有形容词的时候，限定词位于形容词前。例如：a crushing loss.

当一个单数名词被形容词修饰时，有时为了强调该形容词，用程度副词as, so, how, too修饰该形容词, 此时副词+形容词则移至不定冠词之前。例如：as crushing a loss, so crushing a loss, how crushing a loss, too crushing a loss.

再举例如下：

Tom is as clever a boy as John.

Tom is so clever a boy.

Tom is too clever a boy to make such a mistake.

How clever a boy Tom is!

How clever a boy is Tom?