I was in a bookstore and was looking at a manga listed as a josei manga on the back cover before I had to go somewhere else, and am trying to remember the title. I only looked at the first chapter, but these are the details I remember:
The protagonist is a woman who seems to have issues related to body image. In the beginning, it seems that she eats very little for her breakfast and hopes that it'll be enough for later, but in a later panel we see her eating a lot of food.
Her coworkers comment on her fatness at some point in chapter 1.
Her boyfriend's name is Saito. If I'm not mistaken, he's cheating on her with one of the protagonist's coworkers. She discovers this when she sees them on the street together, and later asks him if they can go out together to some place, which he says is boring. She wonders how he knows this detail when he supposedly never went to said place. The coworker apparently went to a Yokohama hotel with the boyfriend and had sex "three times" with him.
We see an image of the protagonist on her knees in the nude to show her having sex (during a discussion of something tangentially related to that with her coworkers) with her boyfriend.
My preliminary searches didn't give anything useful, but I'll try again later.