We've had some success with telling DD what the next small step in a big change, and letting her choose a date for it on the wall calendar that hangs in her bedroom. If need be, I limit the months when she can choose (March or April, for instance). I learned this approach from friends, who found it worked well for their daughter, too.
We've marked the calendar (she chose the date) for No More Nursing in the Middle of the Night, Pee in the Potty or the Toilet Day (after two failed potty training attempts and a child who was 3 years old and had total control but just prefers diapers), and this week, Two Pees in the Potty Day. (From that day forward, she has to do her first two pees of the day in a potty, and after that she can pee in a diaper if she wants to.)
I think it helps her to know she has the power to choose the date and mentally/emotionally prepare herself in advance. We look at her calendar regularly anyway and talk about things that are coming up (grandma is coming tomorrow, music class on Wednesday, playdate with a friend Friday morning, and oh, Sunday is the day you chose for Two Pees in the Potty Day, etc). Once the big day comes, that's the new rule for all future days (so it's really more than just one day, but the calendar day marks the beginning of the new era).
Often she's still sad and resistant when the day comes, but she gets through it. We're compassionate, let her know we know it's a big step and it's hard, but this is the date she chose and we know she can do it. When the calendar declares it, it as if an outside rule was made, and we can be the supportive ones rather than the bad guys. (But we do have to be firm and stick to our guns, even when she gets upset.) In some cases her having sad times about the change lasted a few days, and then she was completely fine and seemed to have totally integrated the new situation into her regular, happy existence. In other cases she just did the new thing with zero complaint, as if it were the easiest thing in the world.
My idea for how to integrate this approach would be to sit down with your daughter, share with her a list of "things that have helped other kids learn how to poop in the potty," ask her for her additional ideas and responses to the ones on the list. (Do you think it would help if I sat with you and held your hand? Or would you rather have some alone time and I could be in the other room?) Have her choose one small step, like pooping in a diaper while sitting on the toilet (but her choice for the small step might be something different). Have her choose a date for this, sometime in the next two months. Follow through. After a week or two, when you feel like this first small step has become easy for her, have her choose a date for the next step (which she can also choose after consulting the list of options you provide) -- maybe within two weeks. It'll be a very slow, gentle process. She'll be in control. You'll support her but also be firm that it's time to learn, that you believe in her and you know she can do it