It's at those times, particularly, that you need to make every effort to turn over your negative emotions. The main reason for this is not just because you'll have a more spiritual focus if you do, or that you'll be a bad person if you don't do it. The most important reason for letting the anger go is much more practical. Anger is a chain which binds you to the person or situation against whom you feel it. You will drag them behind you for the rest of your life, unless you're willing to forgive them.
Forgiveness is the only thing which can dissolve the chain of anger.
It also seems to be one of the hardest things a human being can do. It's even tempting to believe that holding on to anger and resentment somehow validates our pain, even lessens it. In reality, all that refusing to forgive accomplishes is to bind us to the person or event we feel so angry about. In other words, holding on to our anger actually gives ongoing power over us to the very person we dislike so much. We, however, keep the pain fresh in our minds through our rage, resentment, even hatred.