Louis I understand how you feel, really I do. I have been there for so much of my life and one day I just decided enough is enough. My wife and I started to be a bit more open and honest about what we were going through. With each other and our nearest and dearest. It has not solved her depression, but it has made things easier. We are more able to discuss it openly with our family and friends, rather than trying to hide and make up excuses all the time. It did definitely making things better for us. I understand that not everyone is situation where they feel comfortable doing so, but if you do I would highly recommend it.
How is your wife today? Can she talk about these low periods when she is feeling better and able to reflect?