Have you looked up your medication in Dr. Hale' Medications and Mother's Milk? Many medications are safe while breastfeeding, safer than replacing your milk with more formula. Call your local LLL leader or ask your doctor specifically about that book (it is much more breastfeeding friendly and actually has specific information about small studies, half life, etc.). I am bipolar too, so I understand the need for medication (with three kids, oye), and I'm not sure if the medication is not breastfeeding friendly what would be best in your situation, but I will say that it is much easier to have a breastfeeding child. It just makes life easier, an easy instant comfort, less illness, etc. I have been managing mine (which has been much less severe since children, so it may not be comparable to yours) with bioidentical hormones. Maybe something besides the medicine you found to work would work, as our bodies change so much during pregnancy. As the others said, right on for going 6 months, but in MY personal experience (giving leave here that it is very different for other people) mama isn't as happy when she doesn't get to breastfeed.
Good luck, you will do what is best for YOUR family.