I think I know what I'm angry about inside; in fact, I'm sure it has to do with the fact that my parents criticized me as I was growing up. Why doesn't the pain go away?

See Below. Seems like you have an insight into what went on. What people that are close to us do, matters and might affect us long-term. Focus on the present. Psychotherapy will help you with further developing insight and helping you with coping with psychological pain. Keep a diary, which might help with further establishing connections to your past and potentially decreasing pain. Good luck!
Chronic anger. A course of psychotherapy would likely help a great deal. Those wounds can heal. You may fear therapist ridicule making it more difficult to pursue help. This would be "grist for the mill." screening for depression, bipolar disorder, add, anxiety disorders may be helpful as your parents may have suffered from them and it may have informed their parenting - and now your reactions.