As Scotty said, some people don't understand depression and may be turned off because they don't know how to deal with it. I don't know if from what you've said, that I can get that she is doing that. From what I hear, it mostly sounds like your end of things, and I am not blaming you, I'm saying it's the depression affecting how much you are able to talk to her. Not only is it hard for you to talk about anything but the depression but you're refraining from talking to her because you don't want to unload on her, so I wonder if maybe it's just you've backed off and are assuming that she doesn't need or want to talk you as much. I know when I'm down and depressed or feeling negative I make a heck of a lot of assumptions about other people and many many times, I'm wrong in the end about what they were doing.

Here's another way to look at it. She may not know what to say to you first off, but also, you seem to be hesitant to talk to her about your problems and since the depression has taken over so much of your thoughts, you have nothing else to talk to her about. It could be a couple of things. If you're worried about why she doesn't contact you as much, since you may not be contacting her as often as you'd like, she may be unsure if she should respect your isolation. She may not know that you'd welcome her coming to you and seeing how you're doing. Maybe she does care but doesn't know how to pursue communication with you. I think as well as you've gotten along all the years, that it's doubtful that she's written you off or your relationship is tanking.

I think maybe a good talk with her telling her exactly what you've said her about not wanting to dump on her, and burdening her with your problems all the time might help. Tell her that it's just that it's taken over your life and it's hard for you to talk to her right now. Maybe you'll find out that she just hasn't known what to do an this might break the silence between you two. Heck you might find out she's been waiting for you to just ask for her support!

My heart goes out to you, depression is hard. I hope this helps
*hugs*