Is it considered Lashon Hara if I write something about someone that upset me in a diary that I will reread at some point?

Technically, Lashon HaRa, based on the verse forbidding one from "peddling" rumours, is when there is a communication between at least two people. In fact, writing something down where it will be read by no one other than yourself can be a worthwhile tool in combatting the urge "to tell someone" of what happened (compare to Hilchot Lashon Hara 6,4). If, however, reading the diary reawakens the anger you once felt against the person after it has already cooled, you should consider writing letters and then throwing them out after your anger dissipates.