Forgiving others can be the most difficult thing to do, especially when someone hurts you so deeply. Consider what happens when holding on to the anger and hurt. All you're doing is hurting yourself. Someone put it so perfectly saying, "holding onto anger is like holding fire and expecting it to burn the other person. The only one you hurt is yourself." Holding on to anger affects you emotionally and physically, causing physical pain in the form of illness, anxiety, depression, among other things. Even though it can be the most difficult thing to do, it is also the most rewarding, to let go of the hurt, the pain, and free yourself from those shackles you've kept yourself in.
How do you know if you've forgiven or not? It is more than simply telling the person you forgive them, it comes from within you. If you still feel pain and hurt when thinking about that person and what they did to hurt you, then you have not truly forgiven them.
To start on the road to forgiveness it's easiest to understand that the person that hurt you was in so much pain themselves that the hurt they caused you was not on purpose. Liken it to a runaway train. The train is flying off the tracks and will crush anything in its path, you just happened to be in its path. Understanding this can be the first step in forgiveness.
Focus on forgiving the person who has hurt you, thinking of them in a positive light. Thinking of the person that has caused you hurt in a positive light will attract more positivity and happiness and in time you will no longer think of the pain and hurt when you are reminded of that person or whatever it was that hurt you.
Meditation is a great tool to use for forgiveness. Focusing on forgiving the person while meditating will bring positive light to a once painful experience.