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Demi Lovato's upcoming Grammys performance will mark the fulfillment of a promise she made to herself after nearly dying from an overdose ... so expect it to be very emotional.

The singer will perform at Sunday's award show ... her first live performance since suffering the OD in July 2018, and TMZ's learned the song she's chosen is deeply connected to that scary ordeal.

Our Grammys sources tell us ... Demi wrote and recorded the song 4 days before her overdose, and the lyrics will reflect her state of mind during that time in her life.

We're told it's a "big ballad style" tune, and Lovato vowed while she was recovering in the hospital for months ... it would be the first song she'd perform at a big venue if she ever got the chance again.

Sunday will be that big moment ... and she's taking advantage of her comeback opportunity. Along with performing the track on a huge stage, our sources say she'll be releasing the single after the show. We're not sure what the title is yet.

Demi's Grammys gig will kick off a music revival for her. As we've reported ... she's also going to be singing the National Anthem one week later at Super Bowl LIV, and she plans to release a full-length studio album in the first half of 2020.