Jelly Roll says stepping out on his wife was one of the worst mistakes of his life ... and, while he wishes he could take it back, he says he's proud of the person he's become in the years since.

The singer-songwriter got real about his mistake during Tuesday's episode of the "Human School" podcast ... calling the affair a really horrible moment for him because he realized he simply couldn't seem to get life right. Gallery Launch Gallery Getty

As JR explains it, he knew he was head over heels for Bunnie XO ... so, cheating on her really hit him hard -- acting as a sort of wakeup call for the star.

Jelly Roll explains he had the affair because it seemed normal ... at the time, everyone he was hanging around was cheating on their wives. He likens it to part of the motivation

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