The diagnosis for Ella's problem would've been easy if everything was black and white but in the end, we discovered that it came from multiple facets. While it was true that she couldn't handle recoil, from the time it was actually real to our current time, there may have been a point where she was able to overcome it and her body just remembered the knee-jerk reaction the first time she felt it and continued as is.
Because as far as I remember, continued practice eliminates certain things you want to get rid of but Ella feeling scared the first time probably dug deep in her psyche—and one of her triggers was the muzzle flash from a gun she probably shot that time.
"Before I answer, what was the gun you shot that made you scared of shooting large-caliber rounds?"