Best Friend emailed me last night and expressed very clearly where she was coming from. She also explained that she knows she can push people away at the very time she needs them.
I probably could do with some time to look into depression and understand her troubles a little more - I'll add that to the very long to do list I have right now. So if anyone has any experience of this I'd love to hear from you.
I am however over the moon that the friendship is savable, though I think it will require a bit of work on both sides. The lesson in here is that putting your anger aside and concentrating on what matters really is the best thing to do.