When I was younger I had very poor self-esteem. That was compounded when my parents would compliment me inappropriately. That is, they never really noticed the things that were important to me, but would just gush (now and then) compliments over minor things that I didn't care about to what even now I think was an inappropriate degree. Also, it was intermittent. One semester I'd be taken to supper at a cafeteria if I made straight "A"s, the next not. And if we did go to the cafeteria, sometimes I was allowed to get what I'd like, other times they'd give me a $2 limit (which, even then, wasn't a lot). It all came across as very fake, as if they were only going through the motions because they thought they had to, which just made things worse as it meant they could not be depended on to be an accurate barometer of "how I was doing" in the world.
The only thing I can suggest is that you foster your daughter's interests in any way possible. If there's a way to provide her with goals to meet so that she can be successful, that would help. Learned competency is a great antidote for low self-esteem.
Good for you for anticipating middle school. Middle school and high school may not be all that tough academically, but they are just brutal, socially.