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Jack Doherty is spilling on his Miami arrest ... telling TMZ the whole thing was so mind-numbingly boring, he’s actually calling it the worst day of his life.

The embattled influencer tells TMZ it was a brutal slog -- first stuck in a tiny cell with a cop for five hours, then shuffled off for a mugshot and fingerprints ... before spending another 10 hours in a big holding room where he could finally make some calls. Turner Guilford Knight Correctional Center

The day didn’t even end there ... the 22-year-old says he got thrown into yet another cell, took a nap, and then got jolted awake by guards who, he claims, were super nice -- even telling him TMZ had already posted his mugshot -- before letting him finally go.

Looks like he might've gotten the memo ... 'cause he says he

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