Don't let a preschooler decide who she's going to be with. She's not capable of making that decision yet. When we went to pick up my stepdaughter last Saturday, she was crying hysterically as if we were carting her off for a Chinese water torture session. Be careful not to argue in front of her, no matter how mad you are or how irrational your boyfriend's ex is.
Children pick up on it and are confused and scared by it.
If you have to, say nothing, show no emotion and just make it through the child switch. Save your breath with the ex. It's not worth it. And besides, if she wants to argue, you'll frustrate her more by not giving her what she wants. So, it may be a good idea to involve the psychologist.