Child purposely peeing themselves

My daughter will pee herself because she doesn't want to stop playing to use the toilet. I've asked her doctor about it, he said its normal for children to 'act out' when a new little one is on the way. But this has been happening for a long time now. It was happening long before I ever got pregnant again. She won't tell me she's peed her panties, she'll just sit in it until I physically force her to get up. I make her clean it up. She still does it. I've tried taking away her favorite things, she still does it. I've tried spanking, she still does it. I've tried timers, rewards, positive feedback and she still does it every day. I know she's not constipated because she poops in the toilet every day. I am about to lose my ever-loving shit. She'll even pee herself in front of her friends.

Has this happened to anyone else? Does anyone have any suggestions?