I don’t know if you have PTSD, but I do know for certain that you have a miserable grandmother. A way to think about it is that you know directly what the cause of your anxiety is and the variety of symptoms that come from it. Let’s start there.
Sorry you had to deal with her for the time you did. But, whatever we label it, there are ways that you can begin to unhook yourself from her pathology.
The first thing I would do is write a very thorough and unfiltered letter to her that you are not going to send. Don’t mince words. Say everything that you weren’t able to say to her when she was being so hurtful. Take your time in writing this. Being able to express what was on spoken is an important vehicle for unhooking from those memories.
I would start there and see how you feel afterwards. If there is some general improvement you may want to start keeping an expressive diary where you could write the thoughts and feelings that come up about her.
See if this helps. If it does you may find that your feelings will lift. After 30 days if you don’t feel a positive shift, you may want to talk to your parents about getting some counseling. But I would try the writing first.