What you are describing is not unusual for someone who has been traumatized by bullying. You need help — both from the outside and on the inside. On the “outside”: Changing schools was an important first step to healing as it took you out of a situation that was traumatizing. That was an important first step. Now you need help with what goes on “inside” your mind.
Here’s why. When someone has been repeatedly hurt, the person’s baseline of anxiety is elevated. It takes less and less to make it “spike” and to push the person into a self-protective mode. The usual reactions are fight, flight or freeze.
Unfortunately, once someone has been traumatized, he or she will go into self-protection whether it is needed or not. Something that reminds the person of the traumatic situation, like a smell, an environment, some careless words, etc., sets it off. In your case, the school environment is a reminder of all the times you felt threatened. Just being there is sending you into self-protect mode.
You need help untangling your present situation from your past experience. This is nothing to be ashamed of. It is common to kids who have been bullied and abused.
Fortunately, there are well-researched and effective treatments for trauma. Please share this letter with your parents and ask them to help you find a therapist who can give you the support and treatment you deserve. It is highly unlikely that you can handle this on your own. A therapist can help you recover from past abuse and go forward in life with new confidence.
I wish you well,