Hi L.: My daughter developed a slight poop in the potty fear. I remember hearing about a coworker's toddler who was having "fear" issues towards household objects (the smoke detector, electrical outlets, etc.) and our health insurance helpline suggested letting the child decorate the object with stickers. So I let my daughter stick as many of her favorite stickers as necessary on her potty (just not on the seat or inside the bowl). I must say it did help alleviate her poop in the potty fear... Although, honestly she didn't get over it she was more like 2 year and 9 months. Your daughter might need a couple more months to embrace it...? I just kept giving the same message and reward incentives (I made a toy chest, kind of like the toy surprises at a dentist office when candy no longer held a fascination) until it clicked.
However, I will also add that we don't have constipation issues. I did notice some children's fiber vitamins a few days ago at HEB (in the organic section, not where the regular vitamins are; I ended up getting the calcium version). It was one of the Animal Parade varieties by Nature's Plus. You might want to start her on some of those (all natural fiber) supplements... couldn't hurt especially if she is prone to constipation. Also, my daughter actually prefers water to milk and juice during meals. Maybe if you up your daughter's water consumption that will also make her more regular. My point is this: My understanding is if your body is properly hydrated (and you are not constipated for other reasons), you physically can't hold poop inside indefinitely. The muscles of the large intestines are designed to move waste through and expel it... preferably with your conscious help but that is not necessary. As long as the wastes aren't hardened, your daughter simply can not hold it inside indefinitely. So if you focus on ensuring she is not constipated, she will have to go ever 2-3 days...
I assume also that, when you refer to potty, you mean her little plastic one? If you are trying to get her to go in the big one, wow... That might be too big of a hill for her to climb initially (e.g., getting over the fear of pooping and the fear of the big potty which is bigger than her and noisy). My daughter is now comfortable on the "mommy potty" at home but still has a potty fear in regard to public toilets.
Also, when she had her 1 poop accident in her princess panties, she watched me throw them away. Message was clear: we throw away the princess panties if you do that. She didn't like that they got put in the trash... She's a bit protective of her princess, dora, mickey mouse, doggy/kitty decorated panties. It worked (so far).
Good luck! -Karen