My sister married into a Catholic family. It was a miserable marriage as her husband slipped into alcoholism. Then he developed delusions and started writing letters to President Carter rudely instructing him in how to deal with Iran. (I actually wanted to do that myself, but didn’t.)
She divorced him. If you ever had any doubts that divorce can be necessary and isn’t always because people just don’t try, this should put them to rest. There was no way she could have helped him. (Her father-in-law had his son committed.)
What’s the problem with getting an annulment. No one in the Church thinks that suffering with a mentally ill spouse is something God wants for anyone.
Annulments are not easy things to obtain. You have to prove there was no “real” marriage to begin with. If the husband could be proven to have mental issues before the marriage, then perhaps an annulment is possible.
Otherwise ...you take your husband in sickness or in health, til death do us part.