I have a few problems with what you've presented here leading me to think it's fake.
You never say a name for your daughter, I get maybe not releasing her full name but even her first name could help people find her.
Not giving her diary with lots of information to many trained police officers and thinking the guy that "let this all happen" is gonna find her just rummaging through the sewers
Your account was made a day ago and the first thing you post about is a dog you lost and then magically a day later your daughter goes missing too? Weird both posts you've ever made are on r/nosleep
Since I've creeping around on your profile you also seem to be replying to a lot of comments and they definitely seem pretty lax and plentiful when compared to how you think a father who just lost his daughter and planned to search for her would be reacting
I was 13 once, as where you, gingerly laying your extremely personal diary on your neighbors trashbin is definitely not what youd want to do. It's ultimately makes 0 sense for her to have left the book behind except to make your story better than "hey dudes my daughter is gone"