I'm the last person to tell anyone about helmets, having not worn one for most of the 51 years I've been riding. In fact I can count on my fingers how many times I've used a helmet since 1971 and have fingers left over.
I can however provide perspective on it from my personal experience. And let me say up front that I'm not discouraging anyone from wearing a helmet and have never discouraged anyone from using them. There is something to be said for even feeling secure. I'm just giving a perspective on it.
In 51 years I've never suffered a head or neck injury and I began training horses to be ridden by in 1971, have worked cattle from horseback, continue to ride the roads and highways and still train (selectively). That is not to say I have not suffered injuries, just nothing on the head or neck.
I can't say why you are getting so many concussions while wearing a helmet, but there is some interesting data about riding injuries. If all injuries associated with riding horses are grouped together the neck and head (they like lumping them together since the numbers are so low) account of about 10% of all injuries. Arms/hands account for the most, followed by the legs/feet, and then the torso (at least that was true several years ago so I can't imagine that it's change appreciably). Now if we break down the injuries by the two most unrelated riding disciplines the injury rate changes. If we remove jumping from the equation the number of head/neck injuries drops dramatically (and it was only 10% to start with). I we look at only jumping the number of head/neck injuries increase significantly (still the lowest %, but it's much higher than 10). Now I've yet to find reliable numbers on the number of "neck" vs "head" injuries, but of the 3 people I know of (meaning they have been friends of mine or family members of friends of mine or someone I was aware of before their injury) who had that sort of injury, and it only being 3 is not very many, two of them suffered neck injurie, one a head concussion. The concussion was hospitalized briefly and had a full recovery. The neck injuries ended their riding days (one was paralyzed and it killed the other) The two neck injuries were both from jumping.
I guess I've been lucky. 51 years and I've fractured one arm, severely sprained two ankles, pulled a groin, a deep tissue bruise on a lower butt cheek (that hurt as bad and as long as a broken rib) numerous regular bruises, and broken more ribs than I can remember....last and worst being in March 2017, 7 breaks spread out among 4 ribs (train enough new horses for enough years and your injuries will start adding up LOL), but I don't wear helmets and have never even gotten so much as a "goose egg" on my head that was riding related.
Now all that being said, I would have to ask how you got the concussion? i.e. what were the conditions/situation? How did your head make impact, what did it impact against, what was the damage to the helmet, etc.....? It is impossible to make an assessment of something without the right information.