How do you forgive someone, when they have done the unforgivable?

my situation is simple, boy chases girl for 3 years, boy get in relationship with girl and things are great in the beginning, girl ponders if boy is dissatisfied with her, boy will not communicate with girl, girl breaks up with boy, boy continue to seek out a relationship with girl, boy never tells the truth with girl anymore, girl realized boy is full of it, girl leaves but soon discovers she is pregnant, boy and girl agree to work things out, things do not improve, girl breaks up with boy finally, but girl continues to sleep with boy, boy continues to pursue a relationship, girl rejects boy at first, girl after a month finally comes around to reconsider a relationship with boy, girl calls boy one random night, no answer, boy calls girl back, boy isn't on the phone , its his girlfriend, girl has no clue during the time they split boy moved on, during the phone conversation girlfriend reveals to girl she is pregnant also, girlfriend tells girl boy tells her everything, girl has no clue whats going on, boy denies to girlfriend and girl, that girl and boy were still sleeping together, girlfriend believes everything boy says ... boy calls girl and still denies everything but apologizes to girl for everything, boy talks of raising the child together, girl tells boy to **** off, my question after all of this is, how do u forgive someone who would do something so unforgivable?