From Longman Dictionary of Contemporary Englishharden your heartharden your heartEMOTIONALto make yourself not feel pity or sympathy for someone → harden
Examples from the Corpus
harden your heart• But difficulties did not harden his heart.• Today, if you hear his voice, do not harden your hearts.• And then Nancy will harden her heart against him.• She hardened her heart and thought she should swiftly make it clear she had not come in search of him.• He hardened his heart and turned on his computer, smiling acidly at the screen.• He tried to harden his heart in advance, but knew it was just a front.• The girl had hardened her heart so much; there was no point in giving her further cause.• When he had left her with a tiny baby, she had hardened her heart somehow. 