Toddler becoming unravelled after birth of her sibling

Hello Sara, I am a mother of two. My son was born a month ago and my daughter is almost 3 and half. She's unravelled and thus so have I.I thought I prepared her but unfortunately for all of us she has a very defiant and jealous streak. I'm not sure who needs the therapy me or her. I feel like I'm failing her as a parent. Her tantrums, defiance, disrespect is making me fall to pieces. I'm shocked and angry. Everyone keeps saying it will pass and she will get used to her new brother but I can't help but think that although the time will pass and she will adjust, something within her is drastically changing and our bond is being affected.I used to watch episodes of The Nanny and think "How could things get so bad?". Well, in the last three weeks suddenly my reality has totally changed to one of those embarassing episodes.Please help.