The Top Student Heiress and Her Obsessive Boyfriend

An 18-year-old interstellar girl returns to ancient Earth two thousand years in the past. In this era, she experiences familial love, romance, and friendship for the first time. The heroine is kindhearted but no pushover. The hero lost both his parents, has suffered constant abuse, and is deeply obsessive. The heroine is the light in his heart. Both leads are each other's first and only love, and there is no angst. This is an alternate-history story, so don't get too hung up on the dates.