HOW DO I FORGIVE SOMEONE I FEEL DOES NOT DESERVE TO BE FORGIVEN?

I am not really much of a biblical guy, I believe in GOD and I pray all the time, but I don't read the bible and I don't attend church... but about 5 years ago I found out my girlfriends ex (also the father of her kids) has been beating her, and abusing her (psychologically and ually)... ever since then I have had EXTREME hate for him... For reasons I don't understand, I feel like I need to find a way to forgive him for everything, and stop hating him (not like him, just not hate him) he is the only person in my life I have EVER actually HATED... But because I have this feeling, I feel like there is a bigger reason behind it that is far beyond my comprehension, so I need help trying to figure out how to forgive a monster like him from deep down in my heart and actually mean it... I can say I forgive him now, but deep down I know I really don't... so for all you people who are BIG religious people and believe in forgiving people of their sins, I need help learning how to TRULY forgive him... Please help me