i betrayed my best friend

hey reddit! idrk how to say this but to sum it up, one of my best friends shared one of their diary entries to me and i showed it some people who weren't suppose to see it. this happened almost a year ago and i feel EXTREMELY bad. The diary entry was about her crush but it was kind of personal. We have grown to be really close friends and she doesn't know but next year we will no longer be classmates and i will see her less often. the solution i thought of was to wait a couple years before telling her lmao. i know a lot of whatever this is sounds immature and that is because im a teenager. can yall give me some advice pls T_T