The Wife in the Attic by Rose Lerner

Goldengrove's towers and twisted chimneys rose at the very edge of the peaceful Weald, a stone's throw from the poisonous marshes and merciless waters of Rye Bay. Young Tabby Palethorp had been running wild there ever since her mother grew too ill to leave her room.
I was the perfect choice to give Tabby a good English education: thoroughly respectable and far too plain to tempt her lonely father, Sir Kit, to indiscretion.
I knew better than to trust my new employer with the truth about my past. But knowing better couldn't stop me from yearning for impossible things: to be Tabby's mother, Sir Kit's companion, and Goldengrove's new mistress.
All that belonged to poor Lady Palethorp. Most of all, I burned to finally catch a glimpse of her.
Surely she could tell me who cut the strings on my guitar, why all the doors inside the house were locked after dark, and whose footsteps I heard in the night...
"It gave me nightmares...a furious, tender, aching,...