Tonari no Shiba-kun wa Misete Kurenai

"I'm living on the green side of the grass."

Successful career woman Sono never let envy drag her down. She had everything she ever wanted, and she'd gotten it all through her own hard work. On top of her growing list of accomplishments at work, her relationship with her boss (and live-in boyfriend) Taiya was going great, with a proposal right on the horizon...until one unexpected twist brought her whole perfect life tumbling down around her. Heartbroken and needing an escape, Sono moved to a run-down little apartment, only to realize it had a secret hole in the wall that looked straight into the neighboring apartment. When she sees what the neighbor is getting up to, she can't seem to look away, and as the days pass she finds herself more and more obsessed with peeking in on her neighbor...

A mature love story about a career woman intent on being "strong and independent," the mysterious boy next door, and their slowly intertwining lives.

(from mangaupdates)