Hi...
My heart ached for you as I read your email. My brother is bipolar (diagnosed about 5 years ago after a VERY bad manic phase). His dear wife has stuck by him all this time, but she's been through many rough days. They live in the same city as my parents, and they support her a lot. Also, she attends counseling regularly (as does he), which I truly believe has been the key to the fact that they are still married. I don't know what he'd do without her.
You are under a LOT of stress (as if I have to tell you that!), and you CANNOT feel guilty for wishing the marriage--or even life---was over. Loving someone who is bipolar is so difficult and draining. I adore my brother, but I can't imagine being married to him! If there's any way you can get some counseling, I would highly recommend it. It's very helpful to talk openly with someone about your feelings and to get advice from a professional on coping strategies. If cost is a factor, you might check online to see if there are any free counseling services in your area. I know that in my community, several of the larger churches have licensed counselors who have very reasonable rates.
Also, my sister-in-law is very good about taking some time for herself every day---a bubble bath or even a quick shower, a glass of wine and a magazine---something, even if it's not for long. It really helps her destress.
Hang in there...You're not alone!