not have the heart to
Be unable to bring oneself to say or do something, as in He didn't have the heart to tell her the cat had died. [Mid-1600s] Also see have a heart.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.