I once heard, "when you forgive someone, it is not necessarily mean you are bound to forget, you forgive for you not for them.

This is soo true.. Most times, the person you're holding a grudge for does not even know that you're angry at them, so, they go on living their life happily while the person that refuses to forgive will keep wallowing in grudges and thinking about how they hate what the other person did to them.