I think you're looking at forgiveness the wrong way. I think you mean how do you forget about someone. It takes time.. lots of time and self evaluation. Look at it as an opportunity to live with yourself comfortably. Believe that you can live with yourself without anyone first. Then, look for someone else.
I will do my best to reply with an educated, humble and honest answer. Ultimately, it is up to you whether you want to listen to my advice or ignore it completely. Sometimes, my advice may be wrong; occasionally, it will be right. Regardless, I want to do my best to give people answers they are seeking.