Potty training nightmare

I am at my absolute wits end with my 4 yo daughter. She hasn’t been in diapers/pull ups for about a year but she keeps having poop accidents. Pretty much every day. Yes, I‘be talking to her doctor and I’m taking her to get an x ray of her belly to make sure she’s not completely constipated. But I know she’s not. When I told her we were going to the “poop doctor” a month ago she was so scared she stopped having accident completely. So in my gut I know it’s totally behavioral/psychological. Now she’s having them all the time again and I am going absolutely insane. I make her sit for 7 minutes, she doesn’t go, goes into her room and poops in her underwear. She doesn’t want a pull up. I am loosing it you guys!

I’ve done:

-immediate rewards (m&ms, chocolate, special time with the toys we use for when I’m nursing the baby)

-bigger rewards but slightly delayed (ice cream trips, milkshakes)

-chart rewards for a big toy or trip

-scheduled potty breaks

-not even address it. Just clean her up and not say a damn thing

I’m totally frustrated and annoyed. I can’t take this anymore. Any words of advice or encouragement or experience would be much appreciated.