Deb, beth is right. You truly don't need to feel guilty.
You see -- you SAW. When you read about the bad mommy taboo, you SAW and UNDERSTOOD. You didn't run away from it, you didn't get mad at me for posting it or others for finding truth in it.
You don't have to get mad at you, either. Truly you don't need to feel guilty, or feel ashamed.
Why? Because...
You really DID do the best that you knew how to do at the time.
Now, you know something more, and your best will be different because of that.
If you were invested in denial, cared more about your 'image' than you care about your friend, were unwilling to see new things, then you wouldn't have been doing your best.
But that isn't how it is.
You immediately tried to take in the new information and opened up to it, you regret not having understood... Deb, this proves! Proves! Proves! that you WERE sincerely trying to do your best before!
I hope you can see this, because it's so clear and true. You loved your friend all that you could. You did everything for her that you could.
Deb, you were the best friend to her that you could possibly have been. You can take comfort from that, because it is true.