We've been dealing with similar situation for past 2 years. Our dog was O.K. with fireworks, thunder and loud noises for the first 2 years of his life. 2 years ago we were playing in the park, he was fetching a ball and hit a water sprinkler. He was in a lot of pain.Unfortunately it was during Chinese New Year celebrations here in China and when we carried him home somebody put a fire cracker into a can and threw it by our feet. He got very scared and unfortunately ever since he's associated the pain and the loud nouise. He's now scared of thunder, fireworks and even rain drops bashing on the window. We get a lot of hurricanes down here so it a challenge.
( I want my old dog back, the one that was never scared of anything
I thought he would get used to it over the time and he recently surprised us. He figured out that if he plays during the thunder storm he's not thinking about it so much and kind of distructs himself. So anytime there's a storm outside or fireworks we have to play hide and seek with a ball or another toy. You can see that he plays very nervously but he insists on playing.
I don't have any real advise for you and would love to hear what a behaviourist says to you. The only thing I would try is slowly putting the harness on and reward her every time she's good. I mean do this step by step. Let her just sniff it first and reward etc. Btw. wearing a collar isn't so bad if the harness fails but I would love to know how to cure dog's fear