Selena Gomez is opening up about her struggle to quit vaping ... revealing her nicotine habit has permanently changed the way she sounds.

The singer shared a candid video on her Instagram Stories, admitting she vaped nicotine heavily and warning fans not to make the same mistake.

Selena said nicotine is incredibly difficult to quit ... adding she wanted to speak out because vaping just isn't worth it.

She also addressed anyone who's criticized her voice, saying she understands if people don't like how she sounds ... but she's not losing sleep over it.

The good news? Selena says she's stopped vaping and is determined to stay nicotine-free ... although she admits the process hasn't been easy.

She also offered some encouragement to fans dealing with their own struggles, reminding them they can get through tough times, even when things feel overwhelming.