For anyone interested in comments from barefoot trimmers such as myself, it’s very helpful to know your horse’s breed/age, which part of the country your horse lives in and whether the terrain is sandy, rocky, dry, muddy, etc. Also how much time your horse spends turned out and moving/grazing, hopefully in the company of other equines.
IME, the issue is rarely one of soft or thin soles, but rather how firmly the coffin bone (CB) is attached to the hoof wall (i.e., health of the laminar connection). The sort of tenderness you describe is often attributable to stretched laminae, which causes the CB to sit too low in the hoof capsule. Taken “all the way” this results in a dropped CB or “sinker”, which is extremely painful to the horse. Any laminar stretching can result in trapping the sensitive corium between the CB and sole, especially as the horse moves over harder or rougher ground or has to deal with the added weight of a rider.
The normal rate of hoof wall growth is roughly twice that of sole, and I’ve seen horses kept on sandy soils that abrade the sole so that it barely needs trimming. But I’d be concerned that an 8 week trim cycle is the result of some combination of significantly reduced growth and/or quite abrasive terrain. Horn growth is always enhanced by optimal blood circulation within the hooves, which in turn is determined by overall hoof shape and a ground parallel coffin bone on weight bearing.
When you look at the soles on your horse’s feet, is there concavity present? That is, does the hoof “dish in” a little if you hold a ruler flat across the widest part, with the deepest concavity at the tip of the frog? If not, then it’s likely that either the CB is too low in the hoof capsule (possible chronic laminitis), the CB has undergone remodeling (bone loss) and/or the walls have been trimmed too short. What does the white line (the narrow light colored band between sole and wall) look like? Is there any seedy toe or separation (dirt line) present before/after trimming; can you easily dig a groove in it with a hoof pick?
Most definitely take a close look at your horse’s diet and ELIMINATE any and all “sweet feeds” and minimize non-structural carbohydrates (NSC)! Your mention of stomach ulcers/difficult weight gain strongly suggest that your horse has some level of chronic pain and probable hind gut problems (such as leaky gut) that will definitely cause/exacerbate hoof problems such as chronic laminitis.
Finally I must disagree with those of the opinion that bad feet are genetic or that some horses can’t go without shoes. Please realize that applying a rigid shoe to a hoof WILL (no maybe’s) decrease circulation, sometimes to the point that it effectively numbs the foot–thus the appearance of “instant improvement”. But of course this does nothing to alleviate the actual damage and can prevent healing. Also I’ve seen horses who’ve been shod with pads and the constant pressure absolutely destroyed their soles–so thin and weak I could draw blood with my fingernail! If you haven’t already, please continue to look for a trimmer who can take your horse barefoot successfully (tried the listings at http://www.thehorseshoof.com?). Even without seeing your horse, I feel your horse’s problem is very likely metabolic in nature, and that needs to be decisively dealt with before barefoot success is likely. Please feel free to ask if you have more ???’s if you find any of this helpful…