Is This Just My Life Now? (Discouraged and Defeated)

Maybe I'm just a bad mom. Maybe I wasn't tough enough. Maybe I'm not teaching my kid the right lessons. I don't know.

My almost 3 year old is the cutest, funniest, moat compassionate little girl around... Sometimes. Lately, she's been demanding and defiant. She doesn't usually have tantrums unless she is very tired, but she does straight up not listen. If you tell her not to do something, she will look right at you and do it anyways. She yells and screams at us. Throws stuff. Hits things. Just overall behaves like a feral child. We've been pretty consistent with time-outs, but she just sits in her room and kicks the door and screams. When she does finally calm down, she apologizes and behaves for a short period, and then she's right back to it. We have tried ignoring her, talking to her, bribing her. We have tried distracting her and praising her when she does good. But once she's in this mode, it's like she's hyper focused on terrorizing us.

Currently, it seems like this is the norm and her good days are far and few between.

I'm sure some of it is because she has a new, three week old sister. Mommy no longer sleeps in bed with her. She doesn't have our undivided attention. I am still recovering from a C-section and can't take her places. But some of it was already happening before the new baby came. Is this just what threenagers do? Is this just how life is going to be for a while? My husband and I are miserable. I am terrified for when I have to have both my newborn and my three year old, on my own, alone for a full day. I just want to hide from her some days. She goes to daycare twice a week and with her grandparents on other weekdays, which is the same schedule she had before the baby came.

I am trying to spend more time with her, but she is exhausting. I am massively introverted and she wears me down quickly. Some days, it seems to help. Other days, it doesn't matter.

I feel bad for feeling the way I do, but this child has been a holy terror lately and I am so frustrated and discouraged. I don't know what else to do.

I feel like maybe I'm just not cut out to be a mom.