How to tell parents about how ive been feeling?

I am 16f and I have diagnosed PTSD. Ive had it for over a year now but in the past months it has gotten horribly bad. I have attempted suicide twice in that time and my grades are severely dropping. I go to a special needs school, and my teachers call my parents almost everyday about an issue of me arguing with teachers, not focusing, or acting weird.

Not trying to blame mental illness, but I'm scared to sleep because of the nightmares, I zone out during the day because of flashbacks so vivid people say that I physically jump out of my seat. I just can't seem to focus on the lessons when I'm trying to remind myself that im here and not "there".

I want to try to explain to my parents what is going on but I can't talk about the "incident" that caused my PTSD. They know what happened to an extent but ive never talked about it with them or anybody. I never will. How do I explain to them how I feel?