I am sorry to hear that. It is not your fault. It is the fault of the bullies who did it and it is the fault of the adults that did not protect you. You were just a child. You cannot be expected to solve something so difficult as bullying all by yourself. Many adults cannot even do that. Apparently, the adults in your environment could not solve it, so how could you be expected to do it?

Regretting it does not help. Regretting it just prolongs the pain. It just keeps the idea alive that it is somehow your own fault and that you had control. You did not have control. However, you do have control now.

If you still feel invisible, do something about it. You do not need to still keep suffering the damage. That is something you actually can do something about now.