Confession of Doting Love From My Best Friend - Ten Years' Worth, I'm Going All In, so Be Prepared

Thirty-year-old Mizuki is single and having the worst birthday ever—her boyfriend cheated on her. After drowning her sorrows in alcohol, she’s unexpectedly proposed to by Soichirou—a university friend and coworker who's stood by her for ten years. Uncertainty turns to fluttering heartbeats as Mizuki’s friend becomes so much more.

https://www.cmoa.jp/title/309643/