I once went to a lecture by Dr Katherina Dalton, who was an expert on the treatment of PND.
Her theory was that PND is caused by lack of progesterone - but that not only could you have low levels of progesterone in your bloodstream, but that you could also fail to be able to absorb and use the progesterone that you do have, IF your blood-sugar levels are low.
She believed that, if your blood sugar dropped below a particular level, the receptors in your body that took the progesterone from your bloodstream and used it, would switch to being glucose receptors - because it was more important that your body made the best possible use of what glucose was in your blood. Even when your blood sugar went back up again, it took several hours for the receptors to switch back to being progesterone receptors - so a short-ish period of low blood sugar could lead to a much longer one where you couldn't use even the small amount of progesterone that you did have.
She recommended that you eat small amounts of carbohydrates throughout the day, to keep your blood sugar on an even keel. She wasn't saying eat more carbs, but suggested you spread out the carbs you do eat throughout the day - having something carb-based within an hour of getting up, and no more than an hour before sleeping, and every three hours in between.
This is a leaflet from the Association for Post-Natal Illness. If you look under the Self-Help section, it says this:
"Do not go on a strict diet or go for long periods without food. Hypoglycemia - low blood sugar - can make things worse for a depressed mother. If you need to diet cut down on sweet and starchy foods and eat plenty of fruit or raw vegetables when you are hungry."
Dreamingalone - I think this is very important advice for you. If you have PND, and don't eat properly, you could get stuck in a destructive loop where your thinking will only become more disordered due to the lack of nutrients to your brain and due to the adverse effects of poor nutrition on your PND.
Conversely, having a strict plan of when to eat carbs during the day could not only help your PND, by keeping your blood sugar up, it could also help you to feel more in control of things. And you don't need to eat a huge amount of carbs each time - a couple of rice cakes, or similar is the amount that I recall Dr Dalton recommending.
And if you can add lean protein to that, and maybe some dairy and fruit and veg, that would be even better.