B50, I have had a lot of personal experience with depression, going back to my 20's ( I'm 45 now). I have siad this before on the forum that the first sign of depression for me is insomnia. One thing about being aware of this enemy ( depression) is that you start to learn how to fight it. You become more savvy with time, but it takes a long time. This isn't an easy fix it ( hard for a man, I know). Speaking about men, I used to see in the hospital late middle aged men, in their 60's or so, succumb to depression for the first time. For their whole lives, they were able to avoid, deflect, cope, etc., but when they crashed, they really crashed. I'm thinking your W may be in this category, obviously not a man!, but a person who kept her vibrancy going for a long time. This transition, then, is really going to hit her, and could possibly turn into a crisis.
The best thing you can do is to be steady ( gosh I just talked about this) and not to personalize. You sound like a mature, compassionate, capable person. Gather as much info as you can, be there for her, but don't patronize. You will have to learn how to take care of you, so start chopping that wood! Think about counseling...for yourself. You are right about the changes that occur in marriage...they can be so unfair and unpredictable at times, but you are forunate to have a good foundation. I think you will rise to this occasion just fine.