Your daughter is lucky that her mum is looking out for her. Start with the basics OP.
Dumb phone instead of smart phone. The more online activity the more likely to develop anxiety, depression, OCD, cyber bullying, body image issues, eating disorder, reduced sleep and quality poor attention and focus.
At 11 when hormones rage for most girls, adding a smart phone to the mix can be the undoing for some. If a smart phone is absolutely necessary to not be left out from friendship group arrangements lock it down and only allow 20 min of basic messaging apps. Absolutely no social media as there is a strong association with SM at such a young age and all of the above. Limit or cut out online games and apps.
Does she get 9-11 hours quality sleep?
Is she eating a healthy varied diet?
Additional Vitamin D is recommended
Is she doing any sports? Does she have any interest or hobbies that don't involve how stylish she looks? Sports is amazing and there is something that suits everyone.
Does she have friends? Even 1 close friend at school can make all the difference. Encourage her to join school clubs. Encourage her to do her best academically, which can boost her confidence.
Being a pre-teen in secondary school is not for the faint hearted, hair rising for us mums. She will get through it. Have you taken her to the doctor to have her checked about her height? If all is normal, she will grow soon enough and that in itself will give her a new lease of confidence.
Be careful what books you get her. The What to do when you worry too much on the almighty girl blog is absolute rubbish, we have it and I threw it in the recycling as didn't want some poor unsuspecting person to pick it up from the charity shop. I felt it would do more harm than good.
You could try doing this together.
https://www.amazon.co.uk/Good-Vibes-Gratitude-Journal-Mindfulness/dp/1677148721/ref=sr_1_4?crid=2G0H8TJ49O0E6&dib=eyJ2IjoiMSJ9.XlKQywQ9tXDsluRjnQ9jNq4AC4l-aqcxIVe1dOHhAajs7Bm1ll0oUXe5HFWW_F54XNGFaHvZFPaOZLzbWWoRSpb8MQC1icapvCB1ksXoZyf_BuoQhU9zHbZIjWH9jEeoDFcVBbh6JVAaV86LRktF2QYCUP1xNXTnhBxicJBG8NKYwzT8Q97CqSFdfyz0EdUuHhYTaZXPHuCtQWWt3UukIhWXC3L3GkImmVcmyeKkmG8.m_O-r8be_vo2DAYcSXnx4rCcowAHDTzCWTG18rIw68s&dib_tag=se&keywords=gratitude+journal+for+teenagers&qid=1761041578&sprefix=gratitude+journal+for+teenagers%2Caps%2C84&sr=8-4
Get yourself a gratitude journal too, sit together and aim to complete a page a day and chat about it. If non of this helps a few sessions with a recommended counsellor might help. But again be careful who you choose. You know your child best and there are a lot of charlatans out there.