Duchess in the Attic (2019)

Blurb:
On the night of her high society debut, an unknown culprit assaulted Countess Opal. That single incident led to the loss of her reputation among the nobility, and she has faced years of isolation and malicious gossip. Then, one day, she suddenly finds herself married off to Duke Hubert! It turns out he needs Opal’s family fortune to save himself from debt, despite his unsavory attitude towards her. Opal’s newlywed life begins in a dusty old attic. Hubert’s mansion is full of hostility for his new bride, and even the staff begin to bully her and spread rumors. On top of that, Hubert’s childhood friend, a young woman called Stella, already resides in the mansion, and everyone utterly adores her! Forbidden from helping to manage the duchy, the stubborn Opal decides to take matters into her own hands. Looked down upon and underestimated, she formulates a plan to outsmart them all. The new duchess’s brilliant reversal of fortunes begins in the attic!

Source: www.anisearch.com/manga/52492