Well, I guess this is what motherhood is about.
One day, you feel like you understood your baby's feelings and gets everything right. You succeed in making your baby smile and laugh a lot, and you get confidence as a mother.
But the next day, you can't get any of it right. You can't make your own child stop crying or understand what is making him/her so upset. And you feel like failing. You feel like you are a terrible mother.