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 continued, Mellia, who believed herself to be worthless, had a fateful encounter with Douglas, a knight from the royal capital who reports directly to the princess. Douglas was looking for a "magical substance" capable of curing the princess of her illness. The curtain rises on the story of the millennial vow!

(Source: Kadokawa, translated)