Dirty Pain

I have recurring angry thoughts about how I was mistreated and unfairly released from a prior employer. I understand these thoughts are only hurting me, but I don’t know what to DO with them when they pop up, even now, five years after the fact. It was a very painful, scary, and sad experience for me. It happened. Period. I can’t make it go away or make them undo the things they chose to do to me. What can I do with these recurring thoughts of bitterness and resentment?