That level of constipation needs actual help, not the LACK of help that the normal doc is giving you. When a doctor is telling you to change your diet WITHOUT asking what that diet is first? They are not treating you, they are slotting you into a box of averages without listening to what you are even telling them. You can't get an accurate diagnosis if they are not doing their job and getting all the facts.
Here's some ideas:
1. Look at your no gluten diet - do you stick to it with the care of someone with Celiac Disease? Like, can't use shared pans with gluten eaters, can't have your food touch gluten, buy foods from gluten free facilities only? Might be worth trying.
You can't get tested for this disease while on a gluten free diet, but one symptom of Celiac Disease is chronic constipation. It's the symptom that doctors who aren't experts are the most likely to miss. It's genetic. It's rarely diagnosed. AND...many times, people who get diagnosed will look back and have older family members who had ongoing bowel and tummy troubles their whole lives, including colon cancer specifically.
But, if you've had chronic constipation, you may need to have professional help because the colon can become stretched and you have to have a special diet and possibly medication to thin things out for a while so that it can get back into the proper shape.
A gastroenterologist can hopefully help with this - I'd try to get referred. There are also other problems that can cause this which NEED a gastroenterologist. Pancreatic insufficiency can cause problems, for example, or bacterial infections in the gut. Low gut flora or bacterial overgrowth.
In any case, this isn't normal, and constipation for this long isn't just in need of more fiber. >:-(
2. Food allergies or intolerances frequently cause constipation. I've seen this most with people reacting to dairy and soy, but I know people who get this from all sorts of things. Corn, a dye, all sorts of things. An elimination diet - down to the bone kind of elimination diet - plus a food journal to track your symptoms can sometimes help you track this down.
Low level allergies will often not cause any hives or rashes, don't test positive on prick tests (this according to my own allergist). But they can cause inflammation and problems in the gut, including constipation. So an elimination diet is sometimes the only thing you can do to figure out what's wrong.