T.R.
Hi A.,
My daughter went through the same thing. Have you ever used the bathroom at Best Buy in Bellingham? OMG! Loudest toilet ever! It even startles me when I use it! My little one was very adamant about using the toilet and never looked back once she started using it, we just stopped flushing public toilets. The portable toilets and toilet seats are also a great idea, and I always meant to get one, but never got around to it. She eventually just outgrew her fear of the loud flushing and we just dont flush at Best Buy lol. She does tend to over-pea the seat because they have that gap in them on public toilets, so she sits sideways on the seat to prevent wetting on her pants.
When my son was this age I found a little seat that fits right over a toilet seat that is just the right size for them. But he eventually outgrew it so don't worry about that. Good luck!
T.