The Doctor longed to tell him; he'd not felt quite so vulnerable in a while, and some naive part of him longed to ask his old friend for help, hoped that he would come and make it all better. But his more logical adult self knew that wasn't going to happen: Koschei was gone. He was dealing with a man who was apparently incapable of feeling empathy.
It's not important. Just promise me you'll do it.
He swallowed hard, trying his best to keep any stray emotions locked inside. It was too late. This was the best he could do.