I would also consider a magnesium deficiency. Ulcer meds deplete magnesium. Her reluctance to go forward and not wanting to be touched are classic signs of mg deficiency. From www.performanceequineusa.com :
•Very tight, sore back not related to activity, fitness level or saddle fit
•Horse never really relaxes
•Cranky about being brushed or palpated especially over the back on either side of the spine
•Cranky about being blanketed
•History of tying up
•Muscle tremors or all over trembling not related to outside temperature
•Requires long periods of lunging before being able to focus on work
•Does not tolerate work well and works up, not down
•Bucks shortly after workout begins, seems fine at first then bucks or balks
•Would be described as 'thin skinned' or hypersensitive to touch
•Chiropractic adjustments, massage and body work do not have lasting effects
•Has difficulty getting round or picking his back up under saddle, moves hollow
•Difficulty focusing on work, poor work ethic
•Can't be still, repetitive movement, weaving, pacing, head bobbing
I highly recommend MagRestore from Performnce Equine for deficiencies. If your mare does have a deficiency, she will need large dosages for a bit to reverse it. The OTC mag supplements dosed at high dosages will most likely cause diahrrea. MagRestore does not. Wonderful product. If anything, it can help with EPSM too.
I would also consider low selenium......my guy got muscle twitching when his selenium was really low.......but he was also diagnosed with moderate PSSM via a muscle biopsy and is unrideable.
If your horse has PSSM/EPSM best way to diagnosis is via a muscle biopsy and that will also give you how much muscle involvement there is which will give you a better indication of whether your horse will be able to return to work.