As a person who has had two girthy horses, I know a fair bit about it, and think you are missing something very important here Jubilee. Yes she sounds sensitive and will need more care when doing the girth up. You keep saying however that she isn't sore, she'd show me otherwise when I'm riding etc... this isn't completely true. I do not know the horse, but I am going to suggest what I think may be going on.
I was taught by a chiropractor a fair deal about "girthy horses" as well as by my instructor... as this is quite common these days. People just expect that they can come along, do up a girth and because they do it all the time, the horse is used to it! I'm not saying in any way that this is you, you sound like you're really taking good care not to rush the girth up. But think of it this way... how would you feel to have something done up around your belly quite tightly day in and out... after awhile, your muscles will begin to protest despite being used to it.... and especially in the colder months.
I suggest you do this with your mare: before putting the saddle on her, warm up her muscles along her back and where the girth will sit... her muscles haven't been worked yet and will be cold. This is why, when you walk her to the arena you've warmed her muscles up and she's not protesting! To warm up the muscles you merely make a fist of your hand and with firm, quick strokes rub along the back about a couple of inches down from the spine, where the base of your saddle will sit on the horse... do this until it feels so hot that your hand gets really warm... do again along the girth line... on both sides of the horse... it's an extra I dunno, two minutes. Ignore her if she goes "what the!? Why are you doing that", they will get used to it. Then put on your saddle, do it up the way you are, and then walk her to your arena etc. The same with the bit, just warm it up for her, no horse (or human) likes their teeth touching something cold!
A lot of people don't realise how important warm muscles are when dealing with horses that is girthy. I had a cold backed horse once who would buck in the first ten minutes and then give me her all.. its all to do with the muscle inactivity and warmth. Obviously even if you think the horse isn't sore, it wouldn't hurt to get her checked over by a chiro... it's really invaluable to get your horse looked at as it only takes one bad turn in the paddock for something to slip out of place.
You can also just lunge her for the first five mintues before tacking up... just let her stretch her back and warm all those muscles up. Really make sure she's feeling good before chucking gear on. And good luck! It does take time for the horse to settle and go "hey this isn't so bad anymore"... they're so used to expecting the "ache" that it takes a while for them to go "oh its gone" lol.
Please let me know if this works, and sorry if I'm blunt in some points, I don't mean to come across that way!
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