Hi @Florencenotflo
I am sorry to hear that your little girl is having a difficult time at the moment. It is so hard to see our children suffering.
Sorry, this is going to be a fairly long post. I hope it is helpful in some way to you.
Firstly, I am saddened to see how many of the responses to your post have focussed on your daughter's body and/or body image. I know they are well-intentioned, but they are the product of a culture of fat-phobia and body-policing which is ultimately really damaging to self-esteem.
I am going to talk about that issue first, and then get on to the more important question of what might help your daughter right now.
My experience: like you, I was on the larger size as a child. Objectively, I was overweight between the ages of about 9 and 12/13 but my negative feelings about my body lasted a lot lot longer than that fairly temporary time of actually being overweight. These came primarily from being steeped in a culture (as my mother was) of being ashamed/terrified of being too large. My mum encouraged me to join her in dieting, aged 13. I lost weight but remained terrified of gaining weight, being seen as fat, or being asked about my weight.
Children of both sexes, like adults of both sexes, come in different shapes and sizes. Being a larger girl (whether you have a lot of body fat or not) is really hard; it often feels that being tall is only acceptable if you are very slim too. But some people naturally carry more muscle and/or fat than others.
My daughter has a similar build to me. Like me, as a younger child she was not a waif, but by no means fat. When I saw her putting on weight at around 8 or 9 (despite eating in a natural way) I was so fearful that this was the beginning of the same 'weight' ie body issues that I had.
Thankfully the big difference for her was that I had by then been working with a family therapist for some years by then, who had helped me see the negative patterns of shame around body size in my family and start to work to changing it. I was able to avoid restricting her diet in subtle or not-so-subtle ways, and allow her to keep eating naturally ie in tune with her hunger and from a wide selection of 'healthy' and 'not-so-healthy' foods. To my amazement I saw that once her periods started at 11.5 years, her body lost weight. Now at almost 13 she is a natural, healthy weight. Slim but not skinny - with more body fat than some of her peers, with more muscle than some of her peers, but very definitely her natural size and shape. That was her experience - of course we are all different shapes and sizes - but I mention this as evidence for the idea that it can be far better to eat naturally/intuitively, and to accept changes in body size and shape as we develop, than make an issue out of what we eat and what we weight, which so often sets people up for a lifetime of shame, food-restriction/disordered eating, and yo-yo dieting.
Turning back to the issue of self-esteem, the reason I started working with a therapist was that my son was struggling with various issues, including anxiety. The therapist opened my eyes to a different way of looking at self-esteem and where it comes from. It's a lot to try to explain here, but in a nutshell the idea is that self-esteem (and long term good mental health) comes ultimately from our relationships with our family - i.e. attachment relationships. Specifically, from seeing truly seen and understood by our parents for who we are, from having our feelings listened too (but not over-reacted to), from having reliable adult structure, but not harsh boundaries. It is simple in a nutshell but much more complex and challenging to achieve, especially when we (the parents) are carrying baggage from our own childhoods.
You mentioned that you had similar struggles to your daughter at that age, and that your parents 'left you to it'. You are trying to do something different but it's really hard to see what to do when you didn't receive that yourself. This was my position about 7 years ago. In my experience, listening to your child worries, sadnesses, fears etc without over-reacting or dismissing/minimising can be much harder than you think it might be, particularly when the issue is something you experienced as a child and were not comforted in.
I wish there was an easy 'go to' book which would I could recommend which explained all this but I feel like I've read millions and none was the absolute nugget. But things like "The Opposite of Worry" by Lawrence Cohen was a good one - about the value of play with parents as an antidote to the stresses of childhood, and the feeling of safety that comes when you can express your feelings to your parents and have them accepted. Theraplay was the main therapy we received, and there is a Theraplay book of parenting which might also help. Writers such as Daniel Hughes and Kim Golding might also be useful if you want to explore more.
You mentioned that your daughter now worries about her tummy; the legacy of a hurtful comment at school. Another way of looking at this is that she is fearful of the feelings of hurt, and perhaps humiliation she felt in that moment. She wants more than anything to avoid feeling that feeling again.
I know how horrible that can be. Even though I can only recall one comment along these lines to me as a child, I still remember the incident with pain.
The long term solution is not to seek to change her so that no-one ever says something like this to her again. It is to help her process those painful feelings from that moment, so that the sting of them starts to fade, and help her to feel the unshakable force of your love for her, your acceptance of her just as she is, whether she is feeling happy or sad, brave or timid, angry or conciliatory. It is that unshakeable sense of being valued, indeed treasured, for being her, that will form the deep well of self-esteem which will all hope will see our children through the challenges they will inevitably face in the future.