I'm looking for a short story written in the first person where a scientist takes an unintelligent man and gives him a wonder drug. The man steadily gets more intelligent throughout the story, but then relapses. His rise and fall is detailed in a diary.
This is certainly "Flowers for Algernon" by Daniel Keyes. From Wikipedia:
Charlie Gordon is a man with an IQ of 68 who works a menial job as a janitor at a factory, and is attending a literacy program taught by Ms. Kinnian. He is selected to undergo an experimental surgical technique to increase his intelligence. The technique had already been tested on a number of animals; the great success was with Algernon, a laboratory mouse. The surgery on Charlie is also a success, and his IQ triples.
He realizes his co-workers at the factory, whom he thought were his friends, only liked having him around so they could tease him. His new intelligence frightens his co-workers, and they start a petition to have him fired. As Charlie's intelligence peaks, Algernon's suddenly declines—he loses his increased intelligence and mental age, and dies afterward, buried in the back yard of Charlie's home. Charlie realizes his intelligence increase is also temporary. He begins researching to find the flaw in the experiment, which he calls the "Algernon–Gordon Effect". When he finishes his work, his intelligence regresses to its original state. Charlie is aware of, and pained by, what is happening to him as he loses his knowledge and his ability to read and write. He resumes his old job as a janitor at the factory and tries to go back to how things used to be, but he cannot stand the pity from his co-workers, his landlady, and Ms. Kinnian. Charlie states he plans to "go away" from New York. His last wish is for someone to put flowers on Algernon's grave.
Concerning how the story is told:
Both the novel and the short story are written in an epistolary style collecting together Charlie's personal "progress reports" from a few days before the operation until his final regression. Initially, the reports are filled with spelling errors and awkwardly constructed sentences. Following the operation, however, the reports begin to show marked improvements in spelling, grammar, punctuation, and diction, indicating a rise in his intelligence. Charlie's regression is conveyed by the loss of these skills.
If you are looking to revisit it, note that Keyes later turned the short story into a novel, which I think holds up very well even if you read it as an adult.