I hope you don't mind me posting here as i've only made a couple of posts previously.
I had a similar thing happen with a gelding years ago. he was beautiful with his normal rider and a complete twit with me (bad choice of words).
In the end, i had to go back to his previous owner, measure the bit he was used to and then buy that. then i did a measure of her saddle, put it back on him, took photo's and then had to find one to suit. after all that, i went and had 'hand' lessons. (not sure of your terms for it, or even what they really called it but it was all about softening, lengthening, etc of the hands). and i spent hours upon hours watching a video of her riding him to perfect my technique to suit him.
after all that, i got a massage therapist out and found out that with the combination of the new gear, my inexperience with him, and him being 'out' in the poll and just above the wither, it was making him incredibly uncomfortable.
it took about 18 months in total before we were ready to show. we did get to the show, and he went beautifully. (he was the most well rounded boy that day..........unlike at home lol) and we would have placed had i not fallen off (cant even remember why i fell off now).
i had to retrain myself to ride like her and ride in her gear before we could work in harmony. over time, i adjusted him to my riding style and we went on beautifully from there.
hope this helps and sorry if i shouldn't have posted in here.