Shuudengo wa Amai Kiss shite

In her fourth year as a working adult, Saeka is seen as dependable and responsible—someone everyone in the office relies on. She can't even let her guard down around her younger boyfriend, and her only outlet for stress is smoking cigarettes secretly. Then suddenly, her former boss—who knows all about the mistakes she made when she was just starting out—gets transferred from Kansai!? He's the one person she absolutely doesn't want to show weakness to, but before she knows it, she finds herself constantly being drawn to him in this bittersweet, heartwarming love story!!

(Source: Kodansha, translated)