心を鬼にする >> Can anyone explain to me what is the meaning of this expression ?
I guess it's like "I've made up my mind to become ruthless (though I don't want to) "?
I'm not so sure so can anyone give me a correct understanding thanks
Your guess isn't too far off the mark. A more idiomatic way of saying that in English is "to harden one's heart", generally in reference to when you need to do something that someone doesn't want you to do, but that might actually be good for that person.
As explained in my personal copy of Daijirin:
[心]{こころ}を[鬼]{おに}にする
気の毒に思いながら,その人のためを思ってやむなく厳しくする。「―_して破門する」
Relevant entries in Kotobank (all in Japanese), and in bilingual Weblio.