You actually have a lot of options. There are several newer atypical anti-psychotics which are weight neutral for a lot of people and help with both depression and mania (Abilify and Latuda are the two that come immediately to mind). Geodon is a slightly older one which is also weight neutral, but not as good against depression. All the old typical anti-psychotics (Haldol, Stelazine, etc) are weight-neutral and excellent against mania and psychosis, though won't help with your depression much. Lamictal is a newer mood stabilizer which has very minimal side-effects compared to lithium. It's not as good against mania as lithium but excellent against bipolar depression and could help you a lot in combination with something like Geodon to prevent the highs.

In short, there's no need to despair for your stability yet.

I'm sure the diabetes is scary, but I've been watching my mother deal with it for 20 years and once you get used to the routine of testing and taking your medication it's really not so bad. Even insulin these days is pretty painless to administer (and you may not need that, depending on how severe your diabetes is).