Hey J.! Hang in there! My 4th was the same, sweet one minute and then , I would be asking my self where did she come from? She is 5 and adorable now, but still fiesty and opinionated. You sound confident in your mommy skills, so believe in yourself. That is the one thing I tell my friends, (more than offering them advice they may or may not want,) is "TRUST your mommy gut!" . You know your daughter best, and you know you are doing your best, so have confidence in that.
One of the best things I read was, We spend our child's first year of life teaching them the world revolves around THEM. IE they cry, they get picked up, the hungry cry gets them food, they smile, we laugh! THEN we spend the whole second year teaching them the exact opposite! Wait, mommy is on the phone, don't touch, Stop running! Share... and so on. From a little person's perspective, it made it easy for me to see why they would get frustrated. I think I have been pretty good about NOT breaking down and giving in, standing firm about what is nice and appropriate, and my girls all get this now.
Best wishes to you and yours! Trust yourself, and you "mommy gut!"
C., mom of 4 beautiful girls, new to CA