Where do you work? First, see how the meeting goes. Then, I would recommend doing a little research about the American Disabilities Act. You may find that certain laws protect you from being fired or disciplined by an employer who "regards" you as having a disability based on how they treat you (for example, if they force you to take time off for a back injury because they think it makes you less productive at your job, even if you doctor says your back is ok enough for work). If you have been diagnosed with PTSD, this may protect you in that case as well. I am not super knowledgeable about this law myself. I just happened to learn about it by chance a couple times, in a pre-employment training, and when I was having a medical issue in school. Do not mention this ADA law to your employer tomorrow though. See how it goes, and make sure you have your ducks in a row, and try to handle this on your own, before you try to do anything about ADA. But I think you should know what it says so you can assess your manager critically and be certain that you rights as an employee are not violated.
The way your manager has responded does not seem fair. You probably do need to learn to see your flashbacks coming, and learn how to isolate yourself in a quiet place, and avoid upsetting coworkers. Your coworkers do deserve the effort of keeping your personal troubles to yourself. But your manager needs to help you too. When you have this meeting, try to ask her for practical suggestions. Where is a place I can go when you feel an attack coming on? Can I break up my lunch break and take smaller breaks during the workday? Can I use sick leave to take a half day if I am falling apart and if I can find someone to cover me? This way, you can show her you are willing to adapt and perform better for her.
I do not think it is a sign of a good manager if she reprimands you for having a panic attack. That is a cruel thing to do. Intimidation is no way to stop your panic attacks. However, I think the best thing you can do for yourself, is show her you are willing to work toward a solution, and ask for her assistance. Sounding whiny will not help you. Try to show a can-do attitude.
As for grounding...I found that a quick walk outside, or running cold water on my hands in the bathroom, or taking the long way around the building while routing paperwork, and listening to various music at my desk (from nonverbal meditation music to Queen, depending on whether I need to calm down or wake up) helped me get through an internship last fall. When I really felt like falling apart, I used to work on a project in the control room, which is a locked room with no windows that usually only has one other person inside at a time, or is empty. I would also sometimes hide at the bottom of a stairwell, or in the restroom, for a few minutes at a time to catch my breath. I am very good at faking normalcy, so no one ever noticed. Also, pressure tends to keep me focused, and I was very busy the whole time. My worst flashbacks happen on the weekends when my workload lets up. They were never as bad as yours in front of anyone. You sound like you are not so lucky as me. But maybe you can think of similar ways to cope.
What is the worst that can happen? You may need to find another job. Give your manager a chance, but if you feel her treatment of you is unfair, or the work environment is too troubling, you deserve to find yourself a better job in that case.
The best that can happen, is you and your manager will work out a solution to help you adapt.
Whatever happens will fall somewhere between these two extremes. You can probably handle either situation, or anything in between, if you focus on the immediate concerns and practical solutions. Try to feel optimistic about that, and try to have a positive attitude at your meeting. Also, do not apologize excessively. Say something more like, "Your concerns are valid, and I am also unhappy about how my personal troubles are affecting my coworkers, and I would appreciate your assistance to help me behave more professionally." And any negative feedback she gives, try to avoid taking it too personally. Whatever she says really will not be a personal attack, because she does not know you personally. Accept that she may not understand or respond in the best way, because she probably doesn't know any better. Try to figure out what you need and respectfully ask for support. This can still work for you without anyone needing to understand the nature of your condition.
Try to stay calm and get some rest this evening. If nothing else, that will help you reset and have a calm, professional meeting in the morning.
Good luck!
