I have been working as a Sales Assistant to a Private Banker for a major national banking firm for 4 years. I'm nearly 60 years old, and I also have A.D.D. A couple of years ago, I needed to help care for my elderly aunt & uncle as they were dying. However, I never took more than my alotted vacation & sick time to do so. Since then, my boss has been verbally abusive to me. He doesn't provide the information I need to do my job, and when I ask a question, he belittles me and tells me how stupid I am. I feel that he has targeted me in part because of my A.D.D. This treatment has given me nightmares and has thrown me into depression, which only makes the A.D.D. worse. I have escalated the issue to our H.R. department, and recently, they told me that they regarded the situation as very serious and recommended that I go on paid medical leave (which I have done). Now my employer is backtracking and has told me that I "left my job voluntarily" (although I haven't resigned). I've been told that I can apply for other jobs within the bank, but that the rest is up to me. I don't think I can go back into the job situation as it was, and yet I feel that my employer has an obligation to either correct the work situation or provide a different one.
Can you help me understand what my rights are?

You have a bunch of potential issues going. Why do yo uthink it is your ADD that is the issue? Why would yo uwant to work for someone who belittles you? Sounds like yo uare much better off pretty much any where else.

As for your rights and your employer's obligation, you have the right to work in an environment free of illegal discrimination. That doesn't mean you have the right to an environment with a pleasant boss who treats you well. Your employer needs to make sure you aren't being discriminated against because of a disabilty, and if you are, they must ensure it ceases. They need not fire this other person, make sure they are nice to you, transfer either of you, or provide a new work environment.