I can relate. I have a 4 yr old SD, I love her to death, but when my baby was born in June, my patience with her got very short. It's not her fault, she's very young, was/is jealous of the baby. She pouts any time DH shows any attention to the baby. It's slowly getting better as the baby gets more active and playful. She's started acting out more and getting sassy. Part of it is her age and part is the fact that it was her world for almost 4 years, first grandchild on both sides of her family. Now she has to share the spotlight and she hates it. Things she is perfectly capable of doing on her own, she all of the sudden just can't do it and needs help. She's been potty trained for 2 years and now out of nowhere has an accident atleast once while at our house.
It can be very frustrating because it's not your child. She's (or he) is not your to discipline. I have noticed that it is getting better though. I'm not getting as frustrated as easily with her and we are getting back to playing and laughing.
It's an adjustment on all sides. She has to adjust to not being the center of the universe. I have to adjust to having a new baby and having a love for that baby that I could never describe, without alienating my SD. She's not mine, I love her, but the love I have for her is such a tiny amount compared to the love I have for my own child that I grew and gave birth to.
I guess what I have to say is, it will get easier, you will figure it out, the anxiety will subside and you will adjust to your new life. Good Luck!