I understand your frustration, but have no answer as to whether or not you complicated your back problems with pregnancies. Sure, pregnancy can exacerbate low back pain for a few reasons, but is this what happened to you -- only God knows.
What's important here is that you focus on what you've gained from pregnancy, not what you've lost. I don't mean to sound flippant when I remind you that life can be a compromise and that we must count our blessings. In my own situation, I was able to enjoy a highly satisfying career in spite of my chronic pain condition brought on by spine disease.
Would I have traded it all for being pain free today? No, but some days I'd deal with the devil himself to get a little relief. Chronic pain is like that.
Also, pursue treatment. An interventional pain specialist may have injections or other procedures that can make your life less painful. Other techniques can also help.
Your best bet may be to find a university hospital pain management clinic, preferably one with a school of anesthesia and pain management. Here you'll find a wider set of treatment options for pain relief.
Enjoy your family and best wishes.