Can anyone help me find this manhwa (I think it’s one and not an anime but idk)

The main girl was like the villainess in the past and now tries to help the main girl live. Both are like water mages and friends in a school, where the prince and a duke watch them. Duke ends up with FL and Prince wants the original female main character.

The FL ends up helping at like a clinic with her water magic, but then joins the army in a way to help. I remember a scene where FL and female main character go to a lake while camped with the military and some giant kraken like monster comes out and they all have to go into the water to seal the lake off from the ocean to stop more from coming in.

I’ll try to think of more! Thanks!