The Guardian are publishing a series of articles entitled, "The divorce survival guide" which hosts writers discussing the end of a marriage. I think this one provides a really helpful insight.
"One of the hardest parts of my post-divorce depression was dealing with the feeling that the pain was going to last forever. The hopelessness! The darkness! It engulfed everything: you feel bad, and you will feel bad forever. Your brain simply cannot fathom that it is not the case. You can’t convince yourself of this in the moment, but just let the reality float out there until you eventually feel it: it gets better. Even if all you can do some days is tread water with one nostril above the water, know that there is a shore out there somewhere. You won’t find it; it’ll find its way to you".