Resentment/bitterness is like drinking poison and expecting the other person to die, we heard this at our convention and I personally know this to be true. There is a belief by certain psychologists that depression is anger turn inward, anger that has never been dealt with, and it causes the depression. For the record depression is biological, and brain chemistry is affected, this is just one of thoughts of the reason for the change in brain chemistry.
I have spent a long time being angry about four years to be exact. I was angry at everyone, and anyone, and after a long time I realized that anger was only hurting me. The people I was angry with had no idea I was upset, and after a while I didn't even no why I was angry. I truly don't have anger anymore, and the reason this is so real to me right now, is I have a person in my life who suffers from depression and addictions and is so so angry and blaming others for where they are today. I wish that I could tell them to just stop being angry and it would help them so much.
Anger is a poison that only effects you the person who is angry. I know that it is easier said than done, but I promise you that working out your anger will allow you to have a more fulfilling happy life.