Persephone's assurance did not assure him of anything. It crushed him, but he didn't let it show. She held on to something that will never happen. Demeter will not give up her daughter. If otherwise, she would have done it ages ago, letting Persephone and Hades live in peace. As a fellow god and an immortal, Hades knew Demeter would always, always stay true to her nature, and her nature was to never let her daughter go. Persephone said her mother has to come around eventually, but for someone that has eternity waiting for them, eventually would never come. 

Hades smiled at her words, hoping she didn't catch on to his thoughts. "I believe in you, dear Persephone. Soon, we will be together forever." he said. His words echoed within him, each time hurting even more. "Promise me, though, Persephone. Should your plan not succeed, promise me, that whatever happens, you would come back to me. Please, Persephone. I need to hear it." he said, as he held her cheek on his palm.