Masking hurt

Isn't gratitude the best way to forgive?

Yes, if you can be grateful that the person who hurt you is your teacher and is helping you learn and grow and forcing you to elevate your consciousness above revenge. But that realization may not be possible right away, or for everyone. Otherwise, resentment is dealt with by most people through blaming. It is how they process their hurt, just as others process their hurt with alcohol, or sex or some form of escape. Yet blame and retaliation never release the resentment, they only mask it.