I would say that it very well could be him. You need to get to the bottom of it somehow. First talk to your psychiatrsit and see what they think of the situation. If they think it could be you try the Abilify and your plan. You may need to be a bit sneaky and try to catch him in the act, though. :(
Your story is very simple and happens regularly everyday when a mean husband takes advantage of his wife's illness. He is aborting his marriage with his own hands while he was supposed to honor what he swore in front of the priest "in health and in illness". So i would not give him any importance and look after your health.
As to convincing you that you are mad or the sort is a dirty trick that all unworthy husbands do it. It's two fold, to convince himself before you that his adultery is warranted and for you to accept his sins. These tricks are irresponsible.
My only advise, get rid of him
Abilify would help psychotic thoughts. It can cause temporary movement disorders such as akathesia which can appear to be like restless legs syndrome and can be treated with a side effect pill. However, your husband may be abusing the fact that you have a psychiatric disability and pretending that is the cause of him speaking to another woman. I would still be concerned and speak to someone else about it who can find out for you.