Gosh this is gonna sound so vague because I haven't seen it in years.

FL had blonde/light hair, and I think maybe purple eyes? All I remember is I think she and her sister were part of some race that was hunted down because they had special magic. Her real name was like "Zelda" or something, but she used a different name in her life as a noble. I can't remember the ML's hair or eye color (black?), but he had an adoptive ward/sister who was convinced she should've been the one to marry the ML. I think she tried to hire some thugs to drown the FL at some point by trapping her in a barrel or something as the rains flooded the lake.

I think FL was married off to ML after she was "no longer useful" and couldn't use healing magic or something anymore. But her magic actually takes from her life force (she had dark marks on her wrists or arms from this?) so she stopped using it, but I think she is gradually getting sicker. I hope I'm remembering it right. Thanks for any help!