you will become whatever you cannot forgive 

it's a spiritual truth but practically it's because you focus all your energies on that entity in a negative way and then focus changes your brain

if you cannot forgive Trump then you will become someone who gets really good at tiny epithets to torture your opponents and you see the Democrats doing that now 

if you can't forgive the deranged you will become to deranged eventually

if you cannot forgive the Nazis you will eventually start bombing people in a concentration camp like Gaza