My fiance is bipolar and he went through a similar depressive cycle recently. He went back on meds in January (depakote, wellbutrin) and the wellbutrin stopped giving him the lift he needed, he entered his depression, sleeping all day, no motivation, sadness (for both of us) until he was eventually hospitalized and underwent a medicine tweak (more than a tweak, changed his mood stabilizer and a steady dose of lamictal).
I understand how frustrating it must be to have him only meet you half-way. I know how gut-wrenching it is to see the one you love sink as your husband has. I know the questions you pose to yourself. Can I do this? What will the future look like? Would I leave him if he were a diabetic, etc?
You have to think about yourself as well. Especially if he's not committed to treatment. Please know that you're not alone, but also you have to be proactive for yourself. YOu can't lead him to his pill bottle every evening while neglecting your own, so to speak.
Hang in there and keep us posted.