I have lead a group of girls from 6th-12th grade and as many as 35 at a time. I have seen my girls deal with so much and change constantly as they grow and mature.
I can say that if your daughter is not already involved in activities that help her prove to herself that she is smart, accomplished, and capable of doing whatever she sets her mind to than please get her involved. I want to say however, many of the young ladies in my group had their plates too full and they barely had down time. Only pick one activity at a time.
If she loves to swim get her into competitions with a swim group, volunteering at an animal shelter or stable where there are horses (maybe horseback riding lessons); if a child feels that someone or something (animal) depends on them and needs them it is a real confidence booster. If she loves sports get her involved in a sport, gymnastics, music, art/drawing, if you go to church have her get involved with programs, dramas, musicals or the children's choir. If you get involved in an activity at church and she is able to come along and help with set up, decorations or food. Praise her and point out the areas where she is valuable and talented. There are so many choices.
If she does choose something it is very important that she not be allowed to quit if she starts feeling it's too hard, she can't do it, or any other excuses (even if it is hard for her) finishing what she starts is a huge lesson!!! Girls that already have low self-esteem will start out strong and excited but have a tendency to want to drop their activity in fear they are going to look foolish or fail at it any way.
My mother was not my supporter but she would enroll me in whatever I wanted to do. I would go a couple of times and then quit, I subconsciously think I was going to fail and disappoint my mother anyway so I bailed. I took that same attitude in my adult life. I am now 37 and can say that by the time I was 30 I had nearly conquered those thoughts but sometimes deep down I can feel them every now and then pop back up. My mother had even told me that since I was not good at school anyway, why not just go ahead and quit. In 10th grade I did.
I am so proud that you are seeking advice on how to help your daughter. The world is so inundated with the idea of how girls should dress, look, and act that they tend to forget or never learn who they really are. They are too busy trying to conform to an idea or image. Thank you as one mom to another.