Many people don't realize that a large majority of the pain they experience during a break-up has nothing to do with the relationship they really had. Relationships always end for a reason. It is rarely a complete surprise because things generally haven't been going well for a while.
There is often a long list of what each person did or didn't do that led to all the fighting and hurt feelings. Most people don't want back the relationship they actually had. What they mourn for is the relationship they thought they could have had if things had just been different. But the truth is, that relationship didn't exist. Letting go of a dream can be painful.
When the relationship first started there were expectations set for what it could be based on the good things that seemed to be unfolding at the time. Almost all relationships are great in the beginning-- otherwise they would have never started, but the whole of a relationship is what it was from beginning to end.
Because our mind is trying to heal our heart, the painful memories often get shifted to the background and we find ourselves remembering and longing for the good times. We forget who the person really was and idealize who we wanted them to be.
A good strategy for getting past these moments is to simply write down every painful thing you can remember happening during the relationship and read it over to yourself while making the effort to vividly recall those memories until the painful feelings subside. The point here isn't to stay angry, but to remember the full truth of why the relationship ended. Eventually, letting go of these events will be an important part of the forgiveness and healing process, but in order to let go of something you must first acknowledge and accept that it happened.