Toddler making me miserable

My three year old has been acting super horrible these past months. I did just have a baby two months ago but she’s acting HORRIBLE. She hits kids bites tries to throw this one off the bed yells at everyone screams like someone is hitting her or something does want to sleep at all st night anymore. Idk what to do she tries to hurt everyone. I’m at my breaking point with her I’ve had enough with her I don’t know what else to do to make her behave..

She had her check up yesterday and I told her dr about this but she said it’s because of the baby that she is probably jealous. Also she only fights with kids she knows like family but kids she meets at the playground or something she will play good with and be nice..

So a couple of months before the baby was here she started acting up a little bit ever since she got here she has been really worse. And I can only spend time with her while the baby is asleep but I’m not sure what to do when she’s bad. I try to tell her no that’s not ok but I guess she doesn’t really understand. I mean when she hits her cousin she will say I’m sorry after so maybe she does understand that it’s not right. But idk I’ve put her in her room alone when she acts up and stood outside the door so she wouldn’t open it and come out but she just started screaming super loud…

I’m not sure what to do she’s my first child and I’ve always just told her no that’s not ok but she keeps doing it and now I’ve tried to put her in the room but she yells.