Being girthy and being sensitive to grooming/aggressive to grooming is a massive sign of gastric ulcers.
There are the regular fixes like gastroguard, which require a vet diagnosis, scoping, and paying ridiculous amounts for a month blast with the medication. Or, I have seen results and heard extremely many reviews of the herbal/natural supplement by Dynamite, called Miracle Clay. It is volcanic ash that has amazing calming methods, for poultice, internal medicine, etc. I've taken it personally before, and it is so awesome. It is far cheaper and you don't need a prescription or a scope, which is invasive enought that it's my last resort if they don't respond to the Miracle Clay.
I've seen results within 2 days of starting the clay, horses being completely different in demeanor. It's sweet.
I digress, your horse sounds like it has ulcers. Is he/she competed heavily? Was she before you got her? She's lovely, she looks like a great quality upper level HUS horse.
Think about it!
Owner and head trainer of SE-Wisconsin Horse Care
*Lessons*Training*Farm/Pet Setting*Dog Walking*Equi-Eval*