A key, I think, is this: Be aware of the behaviors he exhibits that played a part in your breakup. Is he still showing such behaviors? If so, and if he remains unrepentant, then you need to remember those behaviors and realize that a renewed relationship is not possible without his sustained change. Even if your heart softens, keep a strong mind regarding what he will and will not accomplish with you. So, I think you can forgive and be rid of any deep resentment you may have and then be wise with regard to his behaviors.
For additional information, see Forgiveness for Couples.