So your ex, the person you poured your heart and emotions into, treated you badly. Here are reasons why you must forgive.
Forgiving this person would help you to be responsible for your own happiness because you have eliminated him/her from the space in your head he/she currently occupies.
By forgiving, you would see this person as a teacher and you would have learnt a few lessons here and there.
You would grow, and there would be no further need to play the victim card.
You would appreciate efforts by other people to make you happy. Little things begin to count more and you appreciate people more.
By forgiving, by releasing people from that head space they occupy, we create a channel for us to also be forgiven for our own actions to people.
Remember, what goes around comes around.