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} }.
>
> {I can't tell you} { {how surprised she was} (that) {she won the lottery} }.

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.

She was *very* surprised, surprised beyond my ability to express her surprise.


The *how*-clause includes "I am" and "we are" and *she was* because "glad" and "doubtful" and "surprised" are not characteristic of the coming here or the legislation passing or winning the lottery; rather they are attitudes of the *subject* of the *how*-clause in relation to those events. **Glad** and **doubtful** and **surprised** are *states of mind*.