I've struggled with anx/depression a lot of my life as well and have often had a similar approach/feelings about it that you do. I feel like I've reached a point in my life, now, where I have finally gotten things under control, so here are just some of my thoughts on the subject.
1. Accept the fact that its something that you have to deal with. I totally agree with mind over matter in lots of instances, but anx/depression in ANY degree is an illness. Just like you might suffer from headaches every now and then or have an old knee injury that flares up. That doesn't make you weak, nor does it release you from having to function in this world, but just as you might need to ice your knee here and there or take some tylenol for your headaches now and then, sometimes you are going to have to take some extra measures to deal with how you feel. THATS OK.
2. Its GREAT that you seem to be aware of where you stand with it. You know its not good for you now. You know its getting to a point of concern. There are lots of people who don't have that level of insight. You are already ahead of the game.
3. There are a MILLION natural approaches to dealing with it. Here are some things that work for me.
-at least 1-2 grams of fish oil daily.
-cod liver oil
-Sam-E when things are feeling extra out of control
-daily serving of rich leafy greens OR nettles tea
-talking to people about it(sometimes we just need to get out of our heads)
-Maca can be really great for that energy lull that comes with depression.
As for PPD, I don't know what kind of care provider you are working with, but most midwives will have a million tricks up their sleeves to stave off PPD naturally. A top one that people swear by is placenta encapsulation. Maca is also great and Milk Thistle can be good for dealing with anxiety.
Last pregnancy for me, I was on a low dose of Wellbutrin. I feel even better this go round with the natural approach. BUT, there are lots of women who need pharmaceutical help. My son is PERFECTLY healthy and if you find you need that extra boost from an anti-depressant, don't worry about it. Your babe, inside and outside the womb, needs a mama who is happy more than anything. Do what you need to do to get there. And in the meantime, be easy on yourself, ok?
Take care and hope this helps!