steel one's heart against

steel (one's) heart against (someone or something)

To stop or keep oneself from feeling kindness and compassion towards someone or something. I worry that all the trauma she's already been through at such a young age will steel her heart against other people.
See also: heart, steel

steel one's heart against

Also, harden one's heart. Suppress one's feelings for, as in He finally steeled his heart against them and refused the loan, or You'll just have to harden your heart and tell them the truth. This metaphoric idiom transfers making something hard to rendering oneself insensible or unfeeling. Versions of it date from the late 1500s. Also see heart of stone.
See also: heart, steel