Carpe firm, saying you're too weak emotionally to have anymore children shows me that you have incredible strength. Strength not to cave in to what others want, strength to be true to yourself.
I suffered terribly with PND after my first, and swore I would never have any more children. It was very hard to be open & honest with DH as he wanted a big family. I gave him the choice to choose life with someone who could give him that. He chose me & our DS. We are all he ever wanted and although it was different from our "plans" he didn't want me to go through any further emotional pain.
You're husband may want more children but I can almost guarantee you he won't want more children at the cost of your health.
Be kind to yourself. Give yourself & your husband time to get accustomed to 3 is your family.
As luck has it, once I removed the pressure from myself of having anymore, I felt so much better. We ended up having a second DS & I didn't get PND, it was perfect.
You & hubby decide your family & don't feel guilty because your fertile & choose not to have anymore children.
Chin up. You will see the light & these dark days will end xxx