Sam, it's clear from the additional details that you shared that you are dealing with a lot yourself, and you are doing a lot right.
One thing you don't seem to understand is that your wife probably needs breaks often and regularly. You mention that breaks don't seem to help for long. It's like eating. It's not a once-in-a-while thing. I'm thinking time alone a couple of times a week, or an hour every night. Something she can count on.
Even for people who aren't grieving and don't have PPD, the toddler years can be rough.
I wish I could wave a magic wand and get you guys through the next few years.
Me: 44 H: 44 Kids: 20, 16, 16, and 10 Together/Married: 22 years H announced he was emotionally detached and considering D: 4/4/16 H announced he is going to try to stay and reconnect: 5/1/16