In addition to getting your union involved, get a consultation with a local plaintiff employment and labor attorney. Many give free consultations (check beforehand) so you have nothing to lose. Other than this site, your local bar association is a good referral source. Many such attorneys take the cases on contingency if they want to represent you.
We do not have an attorney-client relationship. I am not your lawyer. The statements I have made do not constitute legal advice. Any statements I have made are based upon the very limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in California.
Unless your union representative is a lawyer, he/she will never provide you with legal advice. You should consult with competent legal counsel immediately.
Some things you might want to think about immediately are:
1) How is your medical care? Are you receiving competent, professional help? Can you do anything more to help yourself or to help your healthcare providers deal with a condition as serious as PTSD?
2) Assuming your primary caregiver does not believe you can perform the essential functions of your job, has he/she recommended a reasonable accommodation which will not place an undue hardship/burden on your employer? Did you consider and request alternative, reasonable accommodations given the fact that so many employees are expected to do so much more with so much less?
3) Have you considered withdrawing your request for an accommodation or would it be impossible for you to perform your job with or without accommodation(s)?
4) Have you discussed whether additional education, skills or training might enable you to perform the essential functions of your job? Assuming your prior performance has been good and that you have shown initiative in the past, will your employer grant you some time to secure new skills or training with the understanding that the goal is for you to resume performing the essential functions of your job satisfactorily?
5) Have your medical experts suggested any lifestyle changes which you yourself can adopt and which might alleviate some of your PTSD symptoms, help you get a better night's rest, or generally improve the overall quality of your life and concurrent job performance?
6) Have you thought outside the box regarding anything else you might do to help your employer so that they can help you?
These are certainly not exhaustive questions but in today's job market all of us have to think and do beyond what we might have thought was reasonable 10 or 15 years ago. The same applies for your employer. Good luck discussing all of these very complex issues with competent legal counsel of your choosing and best of luck with the PTSD. Hopefully, the condition did not arise within the course and scope of your employment but in these economic times anything's possible.