Welcome to the Beyond Blue forums, I think we all know that feeling. Five years is a fair while though and it would be sad if you lose your cool to something that happened long ago, something you cannot change. Perhaps there is unfinished business there or it was a traumatic relationship, some counselling could help you to let this go.
I practice not losing energy to something that I cannot change and I don't like losing energy (through emotion) to someone that let me down. In those moments try and refuse to go with hate, pull your self up and refocus on something that makes you happy, this person has made you unhappy for long enough. It takes practice!
Talk any time.