Today I'm recommending Shissou Sengen by Kurokawa Yori.
In pure essence, this might be one of the most depressing series I've read. It's not outright sad, per se, however it just details one of the more bleak realities about adulthood and being a working person with nothing fun going on in their life. I read it as a cautionary tale about the cynicism of life, and thought to myself that this series is actually quite good at portraying the character's emotions (or lack thereof), and how cruel the movement of time can really be. And as you read it and follow the main character's thought process and actions, you wonder if there's really any way she can change her life.
I recommend this series, but it can be quite depressing at times. However, I really enjoyed it, but maybe that's because to me a manga like this is fresh despite how suffocating it can be.