Her mom, Sarah Palin, who was in the audience during Monday night's premiere, "is just excited that this is a new, confident me, and she's happy to be here to watch," Bristol told PEOPLE backstage after she and partner Mark Ballas completed a 19.5-point, booty-shaking cha-cha-cha.
As for what brought on this newfound confidence, Bristol says simply, "I think just the age progression [changed me]."
"I was 19 [when I first did Dancing] and now I'm 21, and almost 22," she continues. "I think that if I ever hear anything that's being said negatively about me or my family, that has given me motivation to want to do better."
Plus, she said, she's in better shape than she was during season 11.
"I think I'm just more active and cut out all the crap out of my diet," she told reporters, adding that son Tripp, 3, helps keep her active.
"My kid's a lot more rowdy," she said. "He's huge. He thinks he's a teenager. He's into everything. He's really into old-school stuff like Power Rangers and Ninja Turtles. But he also still likes hockey and motocross stuff."
Added Palin: "He's just all boy, all the time."