Originally, I wanted to write a story of my own, but I didn't think my knowledge of Pokemon was up to par. Anyways, here's a really good story I found on FanFiction.net. There's two parts to it, the second being much longer than the first. It's very sad, so watch out! ALSO PLEASE NOTE I DID NOT WRITE THESE FANFICTIONS. THE RIGHTS GO TO THE THE RESPECTIVE OWNER(S).
Written by a guy named Matkin22:
Part one: Reaching Out (Dawn commits suicide)
Part two: Searching for Truth (Ash discovers a letter that Dawn wrote to him before her death; we find out why she lost her will to live)