Please give yourself lots of credit. You have dealt with far more than most young people ever have to deal with and still managed to get decent grades and to be a support to your mum. I think you are one amazing young woman.
You are having a very normal reaction to an “abnormal” situation. No one is prepared to deal with chronic pain or chronic illness. It often wears a person down. You have done very well to make it this far without professional help and support. I’m surprised and dismayed that the medical doctors didn’t refer you to a counselor long ago.
I can’t make a diagnosis on the basis of a letter. But I can tell you that what you are reporting is consistent with a diagnosis of depression. You may have used up most of your reserves for coping with an extremely challenging situation. Please don’t feel bad about yourself for that. It’s like a battery that has been constantly discharging. At some point, you have to recharge it for it to keep working.
I think you need to see a mental health counselor to talk about all you’ve been through and to renew your inner resources. From all you’ve said, I have every confidence that you will make good use of therapy. Your therapist will help you learn new ways to cope and may help you find some new ways to manage the pain.
I wish you well.