I hear you, OP. I have multiple chronic pain conditions and a toddler. It isn't ever easy. For me, it is comforting to know that I'm not alone and that there are other parents who have made (and are making) it through this too. I also think it is important to accept (and sometimes ask for) help. It is also ok to not do it all. I used to feel bad when people at work on Monday would ask what I did on the weekend when the answer was sleep and try to recover from the work-week.
You may want to check out the book "Cereal for Dinner" about how to cope with illness and be a good mom. I think a good part of it is easing up on yourself. It is ok not to do it all. Sometimes you need to rest or stay home from a "fun" activity because that is what is best for your body and therefore what will make you a better parent.
http://www.amazon.com/Cereal-Dinner-Strategies-Shortcuts-Battling/dp/0312317735. As my preacher said one day, there is a reason the flight attendants tell you to put on your oxygen mask before helping others. You can't help others if you don't take care of yourself too.
It has also been empowering for me to talk about my illnesses. I don't drone on about it constantly but I am not ashamed of it either. It is part of me - and people will either accept me or not.
I know some people have also found the red, yellow, and green light system helpful for letting their kids know how they are feeling that day. Something similar to this:
http://foxxfire.hubpages.com/hub/Its-a-Yellow-Light-Day
Good luck!