Well, I have just gone through this as well and with the AWESOME help from the moms at mamasource. My son just jumped the final hurdle and pooped in the potty the night before last. He was also scared. This time, I was able to catch him just before (as he went to hide) and we sat down on the potty. He immediately said he was done, but I knew he hadn't gone. So, I offered to stay or leave him alone (which is what he would normally say if he had a diaper/pull up on) or do you want me to stay? He asked me to stay. I then asked if he wanted me to sing him a song and he said yes. So I did and tried not to look at him in the eyes because I could see that he was a little embarassed about the entire thing. But I made up a song to the tune of if your happy and you know it and sang it over and over and over and over again until I knew he was done and was ready to admit that he went in the potty. So that is how I got him past that.
Now, with the constipation....I went through that with both boys at the begining. I never did the supositories because the Dr. said it would scare them out of going on the potty, but she said to give them prune juice or prunes. My oldest loved prunes! I told him that they were big raisins and he would eat them up. This obviously made him have to go and also made it softer - no pain. Once they feel pain when going on the toilet then they associate it all with pain (that is what the Dr. told me). My youngest didn't like prunes, but he will drink prune juice straight. He does prefer to mix it with fruit punch or something, but he will drink it either way. When we started potty training I made sure to give him some in his fruit punch at least once a day. The reason for fruit punch is that it is so strong that it covers the taste of anything that you add to it.
Try the prunes or prune juice to soften it and it will also make certain to give her the urge to go. Try the singing thing if she likes you to sing to her. I also put my hand on my son's leg so that he felt me there with him, but I made sure not to look him in the eyes unless he was looking specifically at me.
Good luck - it is a hard hurdle to get over and just because she goes once on the potty doesn't mean she will continue to go. I learned this last night as my son, after going in the potty the night before, went in his pants. He is 2 1/2 and he said he forgot. Oh well - I know he can do it and now it is up to me to watch for the sings and make sure he gets there!
Good luck!
Sam