The hard thing about the books was that it was about two girls growing up and the changes, both to themselves and the people around them while in the movie it was only one girl . The books has a lot of internal dialogs that the lead characters expresses their feelings through. The standard mechanism for this would either be narration or a dairy where the feeling are taken down. But that does not translate well in movies, especially teen movies. Time and attention span is limited. So the decision was to make the two characters into one. However, in the book, both girls were different from each other. Sure, the story was about growing up and both girls did that but in different ways. That made the movie's character slightly muddled.
Transposed were too, the situations that both girls faced with their family and friends. The books deal with the girls' reactions to changes around them, in their own way. The movie keeps the situations that the family and friends, takes the reactions of both girls and give them to the central character in the movie. Which makes it harder to understand and relate to the character's reactions. While each set of reactions match to the given situation, when combined it makes the central character unstable. While not schizophrenia, it does sort come close. This ultimately sinks the movie.
If you do happen to see the How to Deal 'book' on the racks at the local bookstore and have read both books separately, browse through the author's introduction where the author put her 2cents worth on the movie. This is a rarity because most of the time writers are apologetic about the screenplay based on their books because a) they got paid to be nice and b)they pity the person who had to distill their multi-dimension, intellect stimulating writing to something people can sit still for an hour and a half.And they get paid some. Not much though. To give you an idea, try and figure out why there is no Forrest Gump 2. In the introduction, Sarah is honest and offers her story as what they are: source materials.
So if you are either fan of the book or the movie, complete the experience by watching the other. Worth it.