Just poop already

I'm really at my wits end on this one. We are in the potty training stage and things are not going well. First, the good news: Lauren has no problem sitting on the toilet. In fact, she loves to sit on the toilet and read her books. The bad news: She has yet, after a month plus of sitting on the potty, to actually go potty. No pee pee, no poo poo, nothing, nada, zilch. The worse news: She refuses to go poo poo at all. We thought we had this under control by offering M&M's every time she went poo poo. Apparently not. She has reverted to crossing her legs, holding it in, and crying. Our pediatrician suggested we stop trying potty training. We tried, but she insists on going into the bathroom.
The event that brought me to my wits end happened tonight. She's sitting on the potty because she asked to sit on the potty. While sitting on the potty, she began to cry because the urge to go poo poo was hitting. What happened? She managed to hold it in and steadfastly refused to go poo poo. *sigh* Just poop already!!!!