You can forgive when someone has been mean to you and said hurtful things. Time is a big healer and people learn to forget these things as time passes. However, what about the bigger things like a murder, rape and killing? Nobody can forgive such things because the loss is very big and it takes a lifetime for people to overcome such tragedies. Even though you are taught to forgive, do not lie to yourself that you can. If you really have to forgive, then it will come to you naturally at the right time.
On the other hand, no matter what the tragedy is, you need to overcome that yourself first. You still have to leave happily and that is something you should try doing first rather than forgiving the others who have hurt you. This is not the right time to learn to forgive. First overcome the hurt and forgiveness will automatically find its way into your heart.