here's some details i've remembered:

• the fl was cursed to die after 6 months by her ( step? ) mother.
• fl was also framed by her mother to be the cruel person in the kingdom , although it was the mother who tormented and killed the people.
• the mother is a queen, and the fl is the princess ( yes, historical au ).
• she got married to a king from another town, and since she will die after 6 months, she asks her fiancé to divorce after 6 months too.
• she healed his friend's niece, which she wasn't able to do in her past life.
• yes she was reborn right after dying due to her mother's curse.

been trying to find this manga for a while now, and i havent finished reading it. if anyone can help i'd greatly appreciate it :] )

The only point that is different is the niece part but I think this might be Saving My Sweetheart