Confused about book vs. series differences...

I have just finished reading the book, and the differences really confuse me:

  • Why did they kill Lucy's dad in the series? He is alive and well in the book. Why did they not show that CJ had an affair with Lucy's crush? This seems like the core of all Lucy's troubles.
  • Why did the series not have Jackie, Lucy's 4th friend? She has a huge role in the book.
  • The series showed so far that Lucy goes back to have s3x with Stephen during the wedding, but in the book, that's when she is truly liberated from him.
  • They show in the series that she accepts him as he is (even knowing about the accident of her roommate), but really in the book, she does not. It is something that she realizes at the end of the movie and what helps her get free from him.
  • Confused about Lydia. In the book, she is still Lucy's friend, and Stephen is engaged to a random girl. Why bring Lydia into this relationship with Stephen in the series? Why make it look like Stephen was taking some twisted revenge on Lucy?

It feels like reading a completely different story, with different characters and lives (other than Stephen - he is pretty much the same, as all narcissistic sociopaths are!)