S.D.
WOW, bringing back memories of my little girl who is now 10 yrs old. She was very afraid of the potty. She hated the flush for years and it was a challenge to get her to go potty on a real potty. I could not train her on a big potty. I had to get the portable ones for her. We let her decorate the portable potty with stickers each time she went. It was a show-case when we were done. IT was great. But for years I had to flush for her and it was not until about 6 years old when she started to flush her self.
However, I did not start my kids til 3 yrs old. Which was late for many peoples opinions...but it worked for us. I think you are doing a great job and if she is already telling you she needs to go, then that is a great start. Maybe the portable potty will be more HERs and she can decorate it and it will change her perspective and stop the screaming. good luck.