thanks to both of you for taking time to respond.
I understand that depression is not a one-off thing, and that even if it gets better, it will most likely come back at some point. But, I have also had several years where it didn't rear its head.
I am jogging almost everyday now, and yes it does help. As far as sunlight goes, I live in Berlin and winter is coming, so that can go out the window. I work from 10am-6pm. The sun is completely down by 4:30pm.
Some days I can find a bit, but mostly I can't derive any enjoyment from things I normally would. I am a musician, and I don't even enjoy listening to music. Really, I can't find space in my head to get away from my neurotic thoughts for hardly a second. Although its just a hunch at this point, I have the feeling that knowing that my girlfriend wouldn't be sleeping with other people for a bit, as this causes me tremendous mental anguish, would give me the little bit of extra breathing room and focus I desperately need to focus even more on improving my situation.
I understand that by asking her to take a break from seeing them, it would be disrespectful to the other partners. However, as I said I she's said that her relationships with them aren't as meaningful as ours is. She hasn't met either of them more than 5 times. And, hopefully I would be able to be comfortable with her seeing them again sooner rather than later.
The bottom line for me is that I just don't think I have the strength to deal with being open for the time being. I have gone back and forth on the matter in my head so many times, and it keeps leading me back here.