Realistically, we all have some sort of mental problems. Especially during separation and divorce. You think your ex has lost his mind, he thinks your psychotic. Telling people you are crazy is about normal.
He may be setting this up to try to use in court. If he’s telling people about this and going to attempt to use it in court he needs to have the necessary medical documentation to back it up…otherwise, maybe he’s just telling people this so that they will be on his side in all this.
If it’s untrue, if you have not had medical or psychiatric treatment, if these statements affect your job or position in the community, then yes, it is slander/libel. If it’s just your ex spouting off that he thinks you have “mental issues” it’s unlikely that a suit for slander would be beneficial. I think that you could use it for the divorce from Bed & Board case, but only if he’s treating you as though you are mentally ill. If he’s just telling others that you are and is not directing this towards you then I’m not sure it could be considered “cruel or barbarous treatment” possibly the indignity but that would have to be up to your attorney to pursue I would think…
I’m not certain about this because normally Divorce from Bed & Board is a formality. One party has usually moved out and this is just added into the suit for ED or alimony/PSS support.