Is there anything I can do to protect myself at a deposition I have been subpoenaed for?

My husband was served with a subpoena at our home that was intended for me. This subpoena is made out in my former name before I married. The follow up letter form the law firm who had me served is addressed to a "Ms." when the name Anthony clearly designates a male. I have been served with a subpoena to testify at a deposition hearing for a former coworker who was fired from her job. In 2013 I left the same employer to be treated when diagnosed as Bipolar 1 and I do recall writing a letter on behalf of the young lady who is suing our former employer. I don't know how much help I can be as I do not recall all of the information that fueled the creation of the letter. This is playing havoc on my Bipolar Disorder and want this to end. It seems the law office is trying to contact me to confirm i will show up and they seem to want to talk to me (probably to have prior knowledge of what I am going to say). I believe my illness and medication therapy has clouded a lot of my memories and I have suppressed a lot of them due to my emotional and mental breakdown causing me to leave my job. My question is am I forced to have to work with the plaintiff and the law firm prior to my appearance and what ground does my bipolar disorder hold (I do have documentation of the illness).