Selena Gomez -- Anything Justin Does, I Can Do Better ... Tumbles Hard Onstage (VIDEO)

Selena Gomez will tell ya ... that first step can be a real bitch when you're performing in front of 19,000 people.

She had a gig in Tulsa Sunday night, and her attempt to go down a short flight of stairs onstage did NOT go smoothly.

Instead of falling for Justin Bieber -- now she's just falling AFTER him.