Writer Ambroise Bierce once said, "Speak when you are angry and you will make the best speech you will ever regret." That said, it's difficult to not speak a word to a girlfriend with whom you're angry. Since you'll have to interact with her eventually, having a few strategies in mind to keep anger from getting the upper hand will prove beneficial.
Let your girlfriend know that you are angry and the reason for your anger. Saying "I feel upset because you gave me no notice before cancelling our dinner plans" is vastly preferable to pretending that you aren't angry and allowing your anger to seep out in the form of sarcastic remarks, intentional forgetfulness or other passive-aggressive behavior. While this approach is sometimes uncomfortable, in the long-run, your relationship will benefit from the opportunity to address and move past the issue that raised your ire.
Think before you speak, recommends the Mayo Clinic. Before blurting out your hunch that your girlfriend will never overcome her selfish tendencies, consider the helpfulness and impact of your words. If the thoughts you want to share won't benefit the relationship, bite your tongue. Doing so can mean the difference between eventual peace and hearing your girlfriend bring up your hurtful words a year from now, long after you've forgotten your anger.
Own Your Feelings
Even though you may be enraged that your girlfriend carelessly dented the door of your new car, it's not a good idea to say "You ruin everything you touch!" Instead, own your feelings and use "I" statements when discussing the problem. For example, you might say "I am disappointed and angry that you weren't more careful when you borrowed my car." This puts the ball in her court, encouraging her to reflect on her behavior rather than become defensive.
Ask for Space
If you're very angry at your girlfriend, it will be difficult to be in her presence without giving in to the impulse to behave in a way that you find unacceptable. Instead of eating dinner together when you are upset -- increasing the chances that you'll snap at her about the annoying way she chews her food -- let her know that you need some cool-off time. Spend an evening reading a novel in your own space instead. Try listening to calm music and reducing external stimulation to help soothe your anger, recommends UCLA Assistant Clinical Professor of Psychiatry Judith Orloff, M.D., in the "Psychology Today" article "Four Strategies to Cope With Anger in a Healthy Way."