I'm in a very similar position right now, and it's awful. But I just thought I'd share my game plan, since I've been doing a lot of research and putting a lot of thought into it.
First, there don't seem to be any safe mood stabilizers out there. Some women choose to nurse and take them, but they have to monitor their infants carefully. I'm not sure I feel comfortable with the possible risks at this point.
Conventional antidepressants likely won't help you, given your history, and they may actually hurt by inducing mania. That also means a no-go for things like St. John's Wort, sam-e, and 5-htp.
That's the bad news. The good news is, there are still a few things to try:
1) omega-3 supplements. Can't hurt, and may help, and will build your baby's brain. Trials with bipolar patients gave up to 10 grams of the stuff per day. My doc said I can start with two tablespoons a day of fish/flax seed oil and take it from there.
2) exercise. Proven to help. Just hard to get to! I'm enlisting friends to get out with me to at least walk, although I'd like to work in more vigorous things soon. It helps, just not for that long.
3) acupuncture. Another thing that helps for a limited time. My acupuncturist assures me that the effect would last longer if I could come in more frequently, but it's kind of hard to get the child care and the $$$.
4) possibly yoga. I haven't tried this yet, but there's some evidence.
5) bright light therapy (like for seasonal affective disorder). I'm planning on giving this a shot, too.
Ultimately, though, if this combination of things doesn't work for me in the next few months, I plan to wean my dd. It breaks my heart, as I nursed her brother for 2.5 years, and loved the whole experience, but my kids need me to be healthy. That's a real priority. Don't feel badly if you get to the point where you need to make that tough choice.