I am being fired for a clerical error on FMLA, what do I do?

I work for a large company. I've worked here for 5 and a half years. This past year I came into a super debilitating medical issue that was difficult to figure out. Upon figuring it out, I needed to take 6-8 weeks off of work and she would reasses more time off or send me back to work. When my disability department contacted my doctor there was a miscommunication of diagnosis and the claim was denied (currently being appealed). In the meantime I filed for FMLA and it was denied because the dates were not filled in. Because I was still on disability status I was not reciving emails from the FMLA department they were mailing out the information and sent a letter requiring a second form with a due date to the wrong address. Because of this my claim was denied. They are no longer accepting any information or even looking at the medical issue. My supervisor said his hands are tied and he can't do anything (although this isn't true). He has treated me differently from the very beginning of this process over a year ago, and I believe is just waiting for me to get fired. Between the clerical issues and whatever he has against me since taking time off I am going to loose my job. At what point do I hire a lawyer? None of this seems right to me as none of it is my fault. I turned in what I knew to turn in always days before the due date. Even after receiving the FMLA letter in the mail after a due date j immediately got paperwork in so I could quickly resolve the issue. What am I supposed to do? What can I do?