Why is being a mommy so hard?

I feel like a failure as a mommy. Why is it so hard to have kids? What happen to the "cookie cutter" ones?

Katy is absolutely a different child than she was a year ago. Why? Is it because of dad dying? She always seemed fine? She won't talk to anyone, even the socail worker. I know she was close to her papa. Is it the ADHD? Can it make her such a different child? We are really struggling with meds, and finding a good dose? It is expensive though, even though the insurance copay is only $20.00, if you keep changing that or the doses it adds up. They are going to test her for dyslexia in the spring. Will this make her so upset and make homework batteles seem 10x worse? She screams at her sisters constantly, and us. She seems to throw these 2 year old tantrums when she doesn't get her way, which she rolls herself into a ball and cries. It is like a complete meltdown. Her grades are still good, all A's and B's. She is good at school. I worry about Katy everyday. I feel like her self esteem is a little lower than her sisters. Not sure if that is because of some of the girls in her class. I know some of the girls give her a hard time because she is a tomboy, and not a girly girl like them.

Yesterday was just my mommy meltdown day. Sam woke up early and then didn't nap. I saw that she was at her wits end. My mom came over and needed me to take her to the store. Bailey and Sam wanted to go with her. I tried to get Sam to stay here, but she cried and nana gave in. So I knew moms back would be hurting worse if she took Sam, So Katy and I went too. Katy threw a fit because she had to get off the Wii. She cried and rolled herself into the ball again. We get to teh store and she is in a crappy mood, Sam won't sit in the cart. Bailey is fine and just happy go long. Katy keeps asking for stuff. Got really bad when I told her no to a stuffed animal. You have to earn toys, and that behavior wasn't earning it. So almost done, and Samantha just loses it. she is tired and just had it. It was 7:30 at night. She threw a 2 year tantrums. I took her and Katy out of the store with Sam kicking and screaming. Put her in her carseat, which was a struggle. She screamed for a good while, and Katy cried. I stood outside of the car. till mom got there. I just had too much. I put them to bed when we came home.

What am I doing wrong. I don't know what to do with Katy. I feel like I have failed her. What do I need to do? Maybe I need a parenting book?