L.,
You are doing a great job. You and your daughter will succeed it is only a matter of time.
My 5 year old daughter regressed everytime some life change happened (when her sibling was born, when we moved, or even when we went away for a week vacation.) Those channges effected her training, but eventually she became potty trained. She is great at it now.
My 4 year old still struggles with potty training and it is fear based too. I have determined she is "trained" in that she knows all the steps. She knows how to take off her clothes, how to sit and pee/poop; she knows how to wipe, and how to get redressed and wash her hands. She can do it all by herself. But she doesn't like flushing.
When we are home, she does well at using the toilet. But she is terrified of "strange" toilets, partly because of automatic flush toilets through area businesses. They are noisy, cold, and it feels scary. She has a tiny body and the automatic flush triggers while she is still sitting on the toilet. I have talked with her about it and she understands what it does. She knows she won't fall into it. And I have purposefully made the flush trigger while I sat on the toilet so she saw it was not so scary. She is improving, but it takes time.
I have confidence your daughter will overcome her fears with your loving encouragement and patience.