Usually, the best is, "I am so sorry"
and when that isn't sufficient, I take comfort in the following quote I just stumbled upon,
There are some experiences in life they haven't invented the right words for.
"We do not know what to do, but our eyes are upon you."
- 2 Chronicles 20:12b
He knows. Thankful He translates my groanings and the quiet, wrenched aches of my heart.