Credit: ABC

Jitters over the jitterbug!

"Bristol is terrified," a source tells Us Weekly of Bristol Palin, 19, who's about to hoof it on Dancing with the Stars (debuting September 20 on ABC). The mom -- fresh off her broken engagement with Levi Johnston, dad of son Tripp, 20 months -- "isn't naturally outgoing," adds the insider, "and doesn't have any performing background."

Palin's partner, Mark Ballas, 24, even had to quell her anxiety. "Mark told her, 'Just stop and breathe,' in rehearsal, because she was so stiff,"  continues the source.

But Ballas was in need of a chill pill too. The show alum, whose partner Shannen Doherty was booted from DWTS in the second week last season, "was bummed at first. He was hoping for someone more dance-inclined," adds the source. "But he's happy now, because he realized how much press they'll get!"

And Palin's biggest hurdle may not be the dance steps but the daring outfits. "My clothes will be the most conservative of everyone's," Palin tells Us. "That's how I am, and that's how I want to dress. But it'll be good -- and the rhinestones and spray tans. That'll be fun!"