L.,
I totally can relate! This is actually more common in boys - our son did the same thing the summer he was 3 years old. He got so backed up it required an enema at the hospital. It was horrible! Our doctors were adamant with us that potty training is a long journey and the best thing we could do was to back off. We were told that if kids hold their stools in for long periods of time, they can damage their muscles and it becomes a much bigger problem.
We actually had to go back to pull ups and start all over for a while. It was mostly a power struggle between us and him, and he ultimately was winning, so we backed off. They also put our son on Miralax so that he could not hold his stool in. It was messy but he eventually learned that his body had to poop. But, then he eventually got lazy in the pull ups and started to refuse to pee in the toilet too, so we dumped the pull ups.
I actually made up a story that the "pull up fairy" visited us and told us that he was a big boy and ready for big boy underwear. I said she took all of his pull ups away - except for the ones at night [we use Huggies nighttime ones, so they look different] and she left him a wrapped package - it contained underwear with his favorite cartoon characters on them)
We started slowly with scheduled potty sitting times - logical times like after meals and before bathtime (or after bath, as our doctor said warm water relaxes the bowels). We read lots of books sitting in the bathroom, played with puppets, anything we could do to make the time go by quickly and make it fun. We had a timer there and had it be 5 or 10 minutes max with a reward for good behavior.
Our son didn't go for delayed rewards or sticker charts, so we used good old fashioned candy! We kept it right there in the bathroom and it was immediate. We also did not do any repremanding for accidents, etc.... it was all positive reinforcement for going and completely ignoring of the messes (no attention at all, just a quick clean up and move on). However, we did give consequences (taking away privileges) for negative behavior (refusal to sit on the potty at scheduled times, tantrums, etc.....). In the end it all worked out but it was a long road.
Bottom line is I spent a long two years washing out underwear, but now he is five and is off the Miralax, doing great all by himself! He still wears night time pull ups; he's just a "late bloomer" in that area and we've learned that although at the moment it is the worst thing, in the big scheme of life, it's really not.
Hang in there :)