Awaken the Heart by Penny Fairbanks

She believes she is incapable of being loved. He has loved her his entire life. A mysterious letter will force two friends to face the truth hiding in plain sight.

Lydia Dailey has never been described as particularly friendly or bright. Luckily for her, she grew up in idyllic Bainbridge with a tightknit group of young ladies who love and accept her for who she is. If only she could get even one gentleman from the ton to do the same. After four unsuccessful Seasons, Lydia is more convinced than ever that she’s much too dour to endear any man to her. As she looks forward to yet another Season of disappointment, she receives a mysterious letter signed only by a “Lady Swan.” The letter gives Lydia strange advice—“Perhaps the one you seek is nearer than you think”—and instantly sparks an idea.
An impossible idea. It seems to Lydia that this Lady Swan is pointing to her best friend, Sebastian Harrowsmith. She’s never considered him as anything more than that. Besides, they’re complete opposites. Where Lydia is cold and rigid, Sebastian is lighthearted and amiable. They could never make a happy marriage.