Compare:
{I can't tell you} { {how glad I am} (that) {I came here instead} }.
{I can't tell you} { {how doubtful we are} (that) {the legislation will pass} }.
The finite clause ending the sentences complements the middle expression, the how-clause.
That middle expression with its complement in turn complements the main clause, I can't tell you.
The meaning of this statement that "I can't tell you" is akin to "Words cannot express".
In other words:
I am very glad, glad beyond my ability to express my gladness.
We are very doubtful, doubtful beyond our ability to express our doubt.