Light Summer and Slightly Cool

Thousands of years ago, he was a human and she was a beast. When he met her, he was still a child. Thousands of years later, she was a demon and he was a human. She transformed into a human form. When she found him, he was dying. Thousands of years later, she became a fairy, but he became a demon. She waited for him for several lifetimes. In every life, he left first, but it was good, at least he didn't have to bear the pain of separation. She could bear the pain alone, but in this life, she had to leave first. She begged him not to forget her, and she would still be with him in the next life. . .