Horses can't hold their breath but they do tighten their abdominal muscles. So by walking away and maybe brushing the mane or tail and then go back and tighten, might work. Also, I had one once that was cinchy, I would untie her and walk her or lounge a couple of laps and then tighten. Over time I came to find that with her I would saddle her and tighten the cinch a little, then pick her feet, then tighten, the brush the tail, then tighten. SHe seemed not to react as bad. I would also lounge her a few laps before I would bridle her and just before bridling and just before mounting I would finish snugging it up. It sounds like a lot of extra time but it doesn't take me any longer than doing it in the correct order. I have also found over the years that in the beginning I had my cinch way to tight, these days you can easily slide your fingers between the horse and the cinch. Hope this will help and hopefully you and your vet if needed can get it figured out.