Perhaps the key word in Agatho's saying is "tried." That is, "I tried never to go to sleep while I kept a grievance against anyone." The reality is that forgiveness is a process. It happens over time and with much reflection. Forgiveness is forestalled when it is stated prematurely.
This is because forgiveness requires mutual understanding. The offended person must come to terms with the offense, asking, "Why am I really aggrieved?" Agatho tried not to sleep when he was hurt because he needed to truly understand his hurt before he confronted his offender. Understanding our injuries takes time.
This is also why Agatho doesn't let those he hurt rest. He needs to really understand how he afflicted them in order to do what is necessary to be forgiven. He has to know who he wounded so he can heal. He must come to terms with his wrongdoing so that he can learn from it and not repeat it. Forgiveness involves this as well. It takes time. Forgiveness is about speaking the truth to the neighbors we hurt and who hurt us. We may settle our anger before sundown, but if we are really after forgiveness, peace and wholeness then we must prepare for the long haul. That's just common wisdom: nothing worthwhile ever comes quick.