It seems to me that your primary problem is the effects of bullying by the older girl. As a child, you didn’t know how to either identify what was happening or how to get out of the relationship. It is no surprise to me that you have difficulty trusting that other peers won’t treat you as badly. It also doesn’t surprise me that you are so highly anxious when you are with people you don’t know.
You’ve already discovered that you can’t recover on your own. Drowning out the distress with music or distracting yourself by cutting works for a bit but those tactics don’t get to the root of the problem. I think the onset of depression is a reflection of your discouragement. You do need help but you don’t know how to get it since you don’t trust adults to help you.
Fortunately, there is a kids’ helpline in Australia. That may be a good place for you to start. Counselors there are available 24/7 to talk to kids like you and it’s free. The website is https://kidshelpline.com.au
I hope you will take a deep breath and also ask your parents to help you. It might help them understand what you are going through if you show them your letter and this response.
I wish you well.