Jealousy is usually a sign of something else. What are you insecure about? I think the best way to handle jealousy and insecurity is to dig deep and find out what they're really about. When you deal with that issue, the jealousy and insecurity become much more manageable, sometimes evaporating all on their own.
I've never heard the term "ambiguisweeties" but it sounds like they're "kinda sorta dating" but nothing's official. It's possible that's part of what's difficult for you. Not knowing where everyone stands can be stressful.
I agree with your decision to distance yourself when NS was making demands on your time and energy that you weren't able to provide. I've dealt with plenty of "mood disorders" of various kinds, and they're a lot a lot a lot of work. If your own mental health is on the line, you absolutely have to protect yourself. It's the old story about Oxygen masks on the plane.
Something else that just came to mind, and I'm just throwing out ideas, is that you could be worried that with NS's anxiety and depression, your own might get lost in her crises, and that you won't get the support you need from Ontario. That could even be reinforced by the arrangement. i.e. since you have support from Virginia, and Nova Scotia doesn't, then there could be some worry on your part that Ontario won't be there for you because she'll be busy being there for Nova Scotia.
The part you said about ambiguity is probably the nail on the head. Humans tend to fear the unknown more than anything else. While their relationship blossoms and grows, it's inevitable that your relationship with Ontario will change in some ways. Not necessarily bad ways, but change can be scary. So until the dust settles, it's natural to be a little uneasy.
It sounds like you're doing all the right things: talking, owning your emotions, being honest with your partners... Things will most likely work out fine, it's just a matter of letting go a little bit, not worrying that things may be a little bit out of your control, and just keep on talking.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker