In reference to BG's current issues, maybe you've already gone down this "list," but have you really thought about+hashed through what specifically might be stressing her in her world?
[Engage "Story-Time with Wallaby"]
Last summer when I was fostering Lady, Lacey seemed like she was nearly falling apart, for lack of a better term.
She had lower energy than I had ever seen, was less interested in food (though, being "Queen of Fatties", she never actually lost weight), was not interested in rides, kept having ERU flare ups (literally once every 2 weeks or so), and her hind legs were totally cut up from somehow stepping on herself.
I assumed she was finally feeling her 27.5 years of life and went along for a few months, basically preparing myself for the worst. But then, somehow I got the feeling that it was not yet her time. My second though, past her age, was that something was seriously stressing her and causing these issues as a side-effect.
So I started listing everything that could be bothering her and highest on that list was LADY. So I sent Lady back to the rescue (feeling terrible about it, of course, but Lacey comes first). Then, second thing on the list was ERU-related pain. So I started her on a daily low dose of safe painkiller.
And we just kept going down the list, taking things one at a time and solving them "right."
Brought home the goats to keep her company because, though I liked the idea of another horse around, Lacey's dearest friends were (at the time) deer and I figured that goats would probably be enough like deer that she should like them pretty well.
Anyway, I soon had my girl back. Her appetite returned, along with her enthusiasm for being ridden, her ERU episodes have nearly ceased, and she's all over doing really well.
Basically, the moral of this "story" is that the biggest thing wrong was the thing I that thought was the most "right" - Lady. I had no idea until I really ironed out everything I had seen and put the little pieces together.
Another horse is probably not BG's main issue, but try assessing what you've seen from her, especially the little things, and think about what she's "saying" in BG-speak. Is everything peachy-keen, or is there something really wrong that you've missed (from seeing it every single day and figuring it's not a big deal to something really small that just changed)?