Having my body.
I was already 5'8" by middle school (5'11" now) and was one of those girls who went through very dramatic development pretty much overnight. I ended up having very curvy hips and very large breasts, much to my chagrin. As if that's not mortifying enough as a young girl suffering through gym class, imagine having pioneers and elders in the congregation telling you that your figure is tempting the men and that you need to hide it in order to "protect" them. (WTF? Easier solution: Creepy old men should stop checking out 12-year-olds.)
I went through a phase of binding just because they made me so ashamed of myself, but fortunately, I got over that. The next time I got pulled aside a few years later about daring to wear a shirt that actually fit instead of a potato sack, I "helpfully" offered to start wearing maternity dresses instead, since that should have provided adequate coverage and hidden my frame a bit. That shut them up. But the self-esteem damage was already done. :/