This is a question that I receive on a regular basis. You are not alone in this. Please keep in mind that the 100th time a person hurts you may be more painful than the first because of the accumulation of resentment in you. This possible build-up of resentment makes forgiveness all the more necessary. So, I recommend three approaches for you:
1) Persevere in forgiving so that the resentment does not overwhelm you. Forgive the person each time he or she hurts you because of unfair treatment.
2) As you forgive over and over, you will get better at forgiving. Be aware of your growing confidence to forgive and your growing ability, which might mean that you forgive more quickly and with better results each time.
3) Please do not forget that you do not practice forgiveness in isolation of the other virtues. As you forgive, ask for justice, and do so after you have forgiven again so that you approach the person with less anger.