The Other Bennet Sister

I was disappointed by this book, yet i was so prepared to love it! I liked the concept and I loved meeting the characters from Pride and Pejudice once more, but because it's another perspective on an already existing book it should follow the character's personality.
In Pride and Prejudice we learn that Mrs Bennet's least favourite daughter is Lizzy but that she loves all her children and is very proud of them, so why is she suddenly so cruel towards Mary?
Charlotte is borderline mean which she never was in the Pride and Prejudice book and her relationship with Lizzy though less intimate after her marriage with Mr Collins was still pretty close, all that was totally erased here.
And somehow Jane's niceness becomes cold indifference?
But all that could be ok if it wasn't just to prove that everyone is mean and unfair to poor Mary who is such a nice underestimated girl.
Honestly I loved to have another point of view and Mary's life must have been difficult but the book would have been so much better if at some point Mary became aware that her own behaviour was partly responsible for driving people away and that she worked on becoming a nicer person ( not only with better clothes)
Or if she discovered that she didn't really apply the pretty words she read in her books it would have been more nuanced and more powerful, here I just didn't like how she was portrayed as a victim throughout the book.
And this is a shame because the author's writing style is really nice and the idea really good.