How do you forgive someone who has cheated on you?

A guy I was in love with cheated on me, stole money from me and would scream at me. It took us 3 years to be official and I didnt want to lose him. He ended up cheating on me...and i forgave him. Later I found out that not only did he cheat, he was still 'technically' dating his ex girlfriend. A cheater can rarely change. well i was with a guy for like 9 months n found out he cheated on me with a close friend..i felt that they did something but everytime i asked him to honestly answer me he always said i asked too much question n it ended up that he did cheat..n it took him a month later to tell me..most people would have just left them both alone but idk why i took my time to listen to them n tried to act all fine with it...i did eventually get back with this guy n was still friends with this girl..i never really forgave or forgot but i also didnt let it ruin me...but i do believe that a cheater rarely change...cuz this guy just left me for my ex best friend..so yeah...after all i went thru with him they were doin things while we were tryin to get together..but thru this...i guess i never forgave or forget but i just let it go...life goes on n is too short to hold grudges..