Why it is hard to forgive a person who made troubled in you?

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I had a friend that I find hard to forgive to the worst thing she ever done to me. She always put me into trouble situation. I was embarrassed to other people because of her selfishness, greed (for the sake of money) and luck of support to me. I thought she can be trusted as she promised to me that our secret was bolted tightly and will never be revealed. That secret in the end was used against to me. I was threatened if I'm not going to use that secret as by witness for her, then I will be the one will suffer in the end and my family will be involve. I was messed up with unholy dignity. I was in deep troubled at that time. To protect my family that top secret was revealed. The worst scenario, I admitted the mistake which is not mine and not coming from me. Is this a real friend? One day she called me up and asking a favor again that she will use me again for his selfish intention. I never gave her a chance anymore.I denied her request. I've never told her fully how am I mad at her. Only half way of it. When we saw each other she greeted me but I am a bit bitter when I responded to her. Do you think she notices that way I treated her? What am i should do? Thanks advance to those who gives input.