all you want to see is make believe

Why do we cling onto what is no longer ours? What drives us to believe that there is only One? One Way, One Person, One Life.
Why is it we can't let go even after all hope has faded?

Let's take a case study:
In a time long ago, and every moment...
Hyma and Corb were friends for some time before deciding to be lovers. They were happy together, but it didn't last. Their innate differences that had once been attractive attributes soon became sources of conflict, and the day came when they decided to separate. What seemed to be a mutual decision initially soon proved to be two very different ideals.
Hyma had though about their situation very clearly and knew that she could no longer be with Corb. She was deeply saddened by this outcome, but was well aware that there could be no future for them together.
Corb, however, believed that this separation was merely time apart for them to sort through their thoughts, and that it would eventually lead to him reuniting with Hyma.
And so, with their separate beliefs, they had minimal interaction, each believing it was the way to their ultimate goal.
Time passed, and Corb felt that he was ready for Hyma. Upon seeking her, he soon realized things were not the way it used to be. If anything, it was worse, as Hyma became increasingly distant. Desperate, Corb pushed harder, believing that Hyma would come to her senses.
Unbeknownst to him, Hyma had unwittingly fallen in love again with a man named Yul, and Yul was equally enchanted by Hyma. Yul was alike to Hyma in so many ways, and Hyma was very happy as Yul understood her in ways that Corb never could. But Hyma did not know how to tell Corb that she had found another man.
Eventually, her radiance of blooming love was cast upon Corb, and he knew that he had lost Hyma. Bitter with rage, Corb decided to set his wrath upon Yul. He waited for a time when Yul was alone, and was about to commit the deed when he heard Hyma call out to Yul. From his hiding place, Corb witnessed the joy in Hyma's every being as she spoke to Yul, and Yul's equal affection for Hyma. A pain ripped through Corb as he realized that the way Hyma looked at Yul was how she had once looked at Corb, but no longer.

What, then, should Corb do? Believing with his whole being that Hyma is the one for him, would he fight for her? Destroy Yul? Destroy Hyma?
Or let her go?