I am very angry with my boyfriend. Is it better to confront him while I am burning with anger or wait until I cool down?
I think it is best to wait. You may say things while deeply angry that you regret later. He may have to forgive you for how you approached him. Waiting, thinking about forgiveness as a possibility, even trying forgiveness first may be best in this circumstance. The reduced anger may help you think through what happened and what you, realistically, would like to see changed in his behavior and in the relationship.
For additional information, see Forgiveness for Couples.