The singer revealed the new medical drama while explaining why she posted video of herself quietly singing her new single, "I Want Love." Jessie, who choked back tears during the  video, said her speech therapist asked her to record herself singing quietly so they could hear her voice and mark any progress.

Problem is ... Jessie said back in February she started feeling a constant burn in her throat. She said she ignored it and presumed it was fatigue from the studio -- but a doctor eventually told her she has major acid reflux and nodules in her throat.

She said the acid reflux was presumably caused, at least partially, by steroids she had taken to treat her Ménière’s disease, but the root cause is still not clear.

Jessie immediately went on anti-acids and a 2-week run of no talking -- but she says it didn't stop the nodules from forming. It's been an incredible ordeal ... she says 6 different doctors have had 8 cameras down her nose, had to adjust her diet to eat the same 6 foods for 3 months, plus blood work, acupuncture and massages.

All of that seemed to work because the nodules went away ... but she says doctors have told her if she continues singing, they'll come right back.

Jessie's trying to fight through it, though, saying, “I am doing all I can to mend whatever it is that is broken to be able to sing these songs live. I will not mime. I refuse."