Heyyo, thanks all for your advice.
I'm definitely going to try to work on this depression thing before I start seriously carrying through with anything. Even if I'm not "cured", I'm going to make sure I can make her happy to the best of my abilities before I even think of bringing anything up. And if it's been depression to blame, well, I've had it as long as I've known her (4-5 years now) so it's not something new and presumptive.
I do think I'll be bringing this up with her soon though. But even though a poly situation isn't my ideal, I think I'll at least give her that option. It's only fair, given the situation. And it'll give me a way to actually try and "win her back" properly. I lost her the first time because I was afraid to have her compare me to Mr Poly Guy, so it's only appropriate really that it should come to that now. I'd still like her to be monogamous with me eventually, and she'll know that even without me saying anything. But right now, I think she's important enough to me that I'm willing to accept what she's willing to give.
Wish me luck!