This question raises several issues. The most important one may be the definition of mental illness. It is very normal for divorce to elicit strong feelings, often negative ones—and for these feelings to change from sadness to anger, guilt, or shame. It is often typical for divorcing partners to label each other as “bi-polar” as they observe these mercurial feelings. And while rapidly shifting feelings may be one of the symptoms of bi-polar disorder, an accurate diagnosis includes several other important symptoms and can only be made by a mental health professional.
The same is true for the broad category of personality disorders—it is tempting to label a spouse as a “narcissist” if, as is typical during a divorce, they engage in self-centered thinking or behaving. Similarly with borderline personality disorder, characterized by unstable and intense interpersonal relationships, which is also common during the divorce process. But a true personality disorder involves an enduring pattern of behavior, not provoked by a traumatic event such as divorce. So be careful with mental illness labels and try to leave the diagnosis to the professionals.
But what about situations where there is a real diagnosis of mental illness, whether it be depression, psychoses, or even schizophrenia? These are usually very serious conditions, which greatly affect a person’s ability to function in a relationship or in their job. Living with such a spouse can be very difficult, demanding, and unsatisfying.
One of the most important factors is the spouse’s commitment to start and follow through with appropriate treatment, which often involves medication. Sometimes patients resist this—particularly with bi-polar disorder. They may balk at medication, which moderates their “highs” (manic states which may be pleasurable) as well as the “lows” (depressive states). Without medication, the bi-polar condition will probably not improve and may get worse over time, and the same may be true for depression. Psychotropic drugs can greatly improve these conditions, and even very serious disorders such as schizophrenia may improve dramatically with appropriate medication (which can also have negative side effects such as weight gain).
Another promising treatment is cognitive behavioral therapy, where negative thinking is addressed and modified and can lead to significant positive behavioral changes. So, if your spouse is willing to help him or herself with appropriate treatment, they and your relationship may improve to the point that you want to stay in the marriage.
Another option is to consider joint therapy, where both of you participate in learning about and how to handle issues that mental illness presents in the marriage and how to improve the relationship for both of you. The National Alliance on Mental Illness (NAMI) is a nationwide group offering education and support for individuals and families dealing with mental illness, and can be a great benefit as well.
So, of course, the decision is yours whether or not to continue in a relationship with a partner who has mental illness. But I think if you are supportive of your partner as they seek treatment, and educate yourself about their prognosis, you will make a decision that you can accept without suffering needless guilt or personal recrimination.