It was all captured on camera ... country singer Elle forgot a bunch of lyrics to Dolly's "Marry Me" while onstage in Nashville, with the performance quickly descending into a drunken, expletives-ridden mess. But the icon says it's no biggie ... putting Elle's behavior down to her navigating some hard things lately.

Dolly actually had nothing but gushing words for Elle ... telling "Extra" Elle's a great artist and a great girl who had just a little bit too much to drink.

She's also urging us all to "forgive that and forget it and move on, 'cause [Elle] felt worse than anyone ever could."

That might be, but the Grand Ole Opry audience that night certainly didn't care how Elle was feeling at that moment ... as the singer resorted to hurling foul language, and mocking fans for purchasing tickets.

Many vented their anger on X ... complaining their evening was ruined by her horrible, drunk, and profane performance.

Her antics were deemed so outrageous that even the Grand Ole Opry said they "deeply regret and apologize for the language that was used."