I think you need to step back and think about it like this:

Replace you as a child in this situation with a different little girl. This little girl didn't know what to do, had no instructions, was confused about this new thing happening to her body, and was probably pretty darn scared about all this stuff going on and not knowing what to do. Not to mention embarrassed, because even if period are completely normal, her mom didn't do what she needed to do and help her through it.

I bet you'd feel so bad for this little girl. You'd feel bad for her, tell her it wasn't her fault, and she had nothing to be ashamed of.

Shame is a really really terrible feeling, but you have nothing to feel ashamed of. Your mom does, that was really negligent of her. That's her shame, not yours.

Keep thinking about that little girl, and you feel bad for that little girl that that happened to. I do, I think it's sad for her and I hope she feels better someday.