I read After as a teen when it was a Harry Styles fanfiction and thought then that it was a revolutionary love story. As I watch it now, I firmly believe that it should've stayed a fanfiction and didn't get published or adapted.
Though there are slight "enhancements" from paper to screen, that did barely anything to salvage the story. The book was definitely darker, more problematic, and more questionable. As for the movie, well, you can only do so much with crappy source material. Among the things that changed was making the cast more diverse, cropping out the instances where Hardin was abusive and manipulative towards Tessa, and changing the disturbing truth about the bet at the end.
Judging the movie only, the script was terrible, the acting subpar, and the plot barely existent. It's honestly just a cliché filled with tropes and montages to indie music.