Forgiveness and Resentment

Question - What do I do if a friend wronged me several time and I feel that I have a right to be upset with her? I know that anger is a bad middah and I recently learned that a person should actually be grateful to those who offend him because this pain atones for our sins. Should I completely forgive the friend? It is hard to forgive the friend b’emes when I still feel so hurt by her insensitivity.