The mom used to be an actress working for a small independent talent agency. She actually managed to get decently popular and had a few movies starring her (these are the movies she asks Fuyumine to never show Akane btw, if it wasn't clear).
At this time she is working alongside her best friend and the two are very hopeful for their future prospects as actresses.
However one day, during a casual meeting with a director and some staff, she starts getting sexually harassed and it's heavily implied the men there organized the meeting to rape her. Luckily she is rescued from there by her manager and nothing happens, but this was scar number 1 in her actress life, which taught her that there are a lot of nasty creeps working in the industry.
Next, while auditioning for a role, the staff carrying the audition essentially ask her for sexual favors in a roundabout way (sidenote: this is a well known practice called makura eigyo, where talents are expected to perform sexual favors in order to climb the corporate ladder). She of course absolutely refuses and is disgusted at the mere prospect of it and goes home.
To her, the notion of a woman/actress offering her body to get a job is just absolutely sickening. And all the staff at her studio supposedly agree, with them even stating they created the agency to protect women from sexual exploitation.
But this is where it all goes wrong. Later on she sees an ad for that movie she auditioned for and discovers the woman in the role (ie someone who very likely accepted to perform those sexual favors) is her best friend. This is a massive betrayal to her, since she thought the two were on the same term when it came to rejecting sexual exploitation.
She has a huge argument with her friend and her agency's staff after this, asking them how could they do this. That's when she learns this isn't even the first time, and her friend has been doing sexual favors from the start to help the agency and the mom get gigs. The friend does say this was out of her own will (ie the agency never forced her) and that she had to do this because this is all she has left in life (according to the flashback she has no family)
Disillusioned with the industry and how it treated her and her friend, as well as feeling betrayed by the people she believed in, the mom decides to just abandon it all together and swears off all media.
This extended to Akane because she didn't want Akane to know about her traumatic life as an actress, nor did she want her own daughter to fall victim to it by virtue of them being related, since there was a possibility she could be scouted.
This is why the mom is so serious about Fuyumine strictly controlling Akane's involvement with movies.
In chapter 73 Akane finally discovers what happened to her mom as well, but in probably the worst way possible. The mom's old manager just spills the beans out of spite for the mom having left his agency in shambles (since she quit abruptly it ruined a lot of business they had at the time). Thankfully Fuyumine punches the crap out of him.
And that's where we are as of writing this post. Hope that clarified it
