Saint Mellia and Knight of the Millennial Kingdom

Mellia, a girl who can't use magic, lost her grandmother—her only family member—when she was very young, and was then taken in by an abusive household. As the painful days continue, Mellia, who believes herself to be worthless, has a fateful encounter with Douglas, a knight from the royal capital who reports directly to the princess. Douglas is looking for a "magical material" capable of curing the princess of her illness. The curtain rises on the story of the millennial vow!

(Source: Kadokawa, translated)