We got Taylor and an absolute WALL of Swiftees Thursday night in Park City, UT ... where her "Miss Americana" documentary premiered. We're used to seeing her fans show out in force, but you gotta see this ... 'cause they outdid themselves for the Sundance Film Festival event -- and Taylor took selfies with scores of them.

T-Swift was smiling and clearly overjoyed -- moments before going into the theater, where fans got to see her open up for the first time about starving herself while she was on the road. In the Netflix documentary, she says seeing herself in paparazzi photos "where I looked like my tummy was too big, or someone said that I looked pregnant" triggered her to stop eating.

She says the lack of eating started to affect her live performances -- but adds she now understands she's gotta have food to have the energy to get through her shows.

As we reported, Taylor's actually pulling back on live shows this year -- doing fewer huge stadium dates -- partly so she can spend more time with her mother ... who's battling cancer and a brain tumor.