Frankly, the 'devil inside her' premise gets used a lot, and without a specific author or series title, it's a needle in a haystack. My mind jumps to urban fantasy or paranormal romance, where a female protagonist shares headspace with a demonic entity—think early Anita Blake or some indie stuff on Kindle Unlimited.
The main character in those is usually the woman grappling with this internal power, so she's the clear lead. The 'devil' often acts as a separate voice, a mentor, or a tempter, blurring the lines of who's really in control. That internal conflict is the story.
If you're thinking of a particular book, checking the blurb or the top reviews on Goodreads will probably name the heroine right away. Titles with that vibe often have the heroine's name front and center, like 'Seraphina's Demon' or something similar.