My boss took it better than I expected. I was surprised that she was so surprised, actually.
It's such a relief to be through with this part of the process. I haven't made any public announcement yet, I'd like to do that later this week. But my office mate knows--I waited until I had told my boss because I felt I owed her that courtesy. I gave her the letter in person so she wouldn't feel ambushed by it in her mailbox and maybe read it in front of other staff.
While I do wish I felt free to "process" my reasons with her, I don't feel like this is a good time to do so. I do feel that otherwise I have done my best to make this a smooth transition for her and for the company I've given 8 years of my life to.
I hope this turns out for the best for all concerned.