I’m wondering if the OP meant that the mentally ill spouse was mentally ill at the time of the marriage, but it wasn’t discovered until a later time. I think that would make a difference. When you are talking about mental illness, I think it is pretty rare for it to just spur up out of the blue. In rare instances, I think you have a lot more of a chance of being able to deal with the problem compared to something that is long-standing and engrained in them. I’m dealing with a divorce from someone who has a personality disorder due to childhood abuse. From my experience in trying just about everything, there really is no reason to expect that he will ever be better.
Mental illness can make marriage hell. It can have profound effects upon the children growing up. It really is not a matter of “in sickness and in health” as the person who made the marriage vows wasn’t fit to make them. In the case of my marriage, my husband wasn’t fit to make the commitment to “love, honor, and cherish” me. I’ve been told by my counselor that he is incapable of real love.
Here is an excerpt from a good publication you can find here, The Catholic Teaching on Annulment.
kofc.org/publications/CIS/publications/veritas/list.cfm
"However, with some persons, psychological problems have become the compelling and motivating force of life.The sense of alienation or inadequacy,self-depreciation,hostility, sexual problems,impulsiveness or selfishness can be pervasive and chronic.It is most unlikely that such a psychologically burdened individual can establish and maintain the close, empathetic, cherishing relationship with a spouse which provides for mutual growth and the proper rearing of children.To put it briefly,a marriage can be annulled if the person entering marriage does not “have what it takes”to develop the community of life and love that is the substance of the marital pledge.
For some time now,the Church has recognized that psychoses, and such disintegrative mental illnesses as schizophrenia and manic depression,can so impair mental and emotional stability that one’s consent to marriage lacked the necessary discernment or capacity.More recently,using further insights,the Church acknowledges other dysfunctions of personality that may make a particular marriage union impossible. It is impossible to make general statements about such matters, because the human personality is so complex and the circumstances of each marriage vary widely.But with that caution,it can be asserted that conditions such as homosexuality and alcoholism often undermine the capacity for a permanent union.
Another group of emotional disturbances are called “personality disorders.”These may not show the acute episodes or the bizarre features of psychoses, but they are marked by deeply ingrained maladaptive patterns of behavior, usually with roots in early life,and often evident by the time of adolescence. Such persons may function quite well in their work, be excellent providers,efficient household managers,or spectacular entertainers. But they are psychologically unable to meet one essential criterion of marriage: the close and intimate personal relationship of mutual support and affection."