@JMSA There are aspects of what you are sharing that I could have written about one of my daughters. Solidarity.
Mine like yours is such a great kid. She is always kind, inclusive, smart and respectful to others in a way that many peers struggle to be. However, she has these very private MH/anxiety/OCD & self-harm struggles that can cause reactions and behaviours that are unrecognizable at times and that makes it very hard. She can lose her temper so quickly and spiral to places I wish I could better understand and feel like the opposite of who she is. I'm modelling communicating gently, taking space and doing what you can both to help with prevention and support in those hard moments/hours/days.
She is doing private therapy twice monthly and is now on medications. I think your GP Is misinformed about being too young, there are many tweens and teens that benefit from support whether long term or short term. I was hesitant at first but they did help and my daughter was able to articulate she felt 'less frantic inside' which was huge. It has allowed for some strategies to be used, but it's not a quick answer.
I often wonder how long this will last and then I remind myself that for many this is a lifelong journey they have to navigate, plan for and understand about themselves. Just like someone who is diabetic. I am trying not to look at it like a phase, nor see it as a terrible thing she is suffering. It is part of her life but we are working to help ensure it doesn't define her life. My daughter's is rooted in some early traumas, and so we talk about how many great people have trauma that does impact their brains, mental health and development and they aren't weak, nor less deserving of good things in their life.
One thing I will add, is I've come to see it's a bit of a grief for me too. What I mean in that we have these expectations at times of what our dc are like and will be like. I never thought my daughter would steal for example. My value system would never allow me to do that, and that guides my decisions. For her, she's looking to feel better and if that means taking sweets or money, she does it, both from me (money) and from elderly grandparents (money) who are so hurt by it. This was perhaps the hardest part for me, when 'behaviours' aren't reflective of my values as a parent or human. It makes it feel much more like a loss of who I thought my daughter would be. I have found acknowledging this allows me to deal with my expectations and not put that on my DD. It's not fair to, but it's also something I have to acknowledge and not judge myself for feeling. This has also helped me support my DD's siblings who have been hurt by her newer communication style, her ignoring them or raising her voice more easily. Those conversations helped them acknowledge, especially her younger sister, that they felt she wasn't who they love and remember because she is very different now. Acknowledging their pain let us work on new dynamics, finding positive ways to interact and helping everyone better understand how hurtful words can be.
I have no easy answers, but know you aren't alone.