Here is some general info on this topic.
Constipation can be caused by:

A.) decrease your intake of IgG foods and talk to doc about products like Metagenics Bowel products or cheaper Sanar Naturals Colon Cleanser 2002]
B.) bad micro-organism overgrowth (bad bacteria or fungus) -more assoc with diarrhea
C) Deficiencies ie water, fiber or Magnesium (can also cause anxiety and muscle twitches) - if you have chocolate craving this can be indicative of magnesium deficiency.
D.) Decrease innervation of the GI
E.) Medications that can be contributing to constipation ie pain medications. However, I have heard medical marijuana does not have this effect.

So you could talk to your doctor again about IgG food allergy test (ie Life Extension lab has one for 95 foods you can do at home) and eliminate problem foods, avoid toxins and look into probiotics.

Get a magnesium, potassium, calcium and Vitamin D3 (tells gut to absorb calcium) check done by the doctor and ask for the results. If you are deficient, or the bottom 1/3 of population, you may want to increase your numbers to a healthier part of the normal range. You need these 3 minerals to be able to innervate the nerves. If they are low they can be further decreasing your innervation to your GI. You already may have damaged myelin on the nerves innervating your GI. You don't need anything else slowing this action on top of it.

If all else fails you can look into coffee enemas. See 'The Gerson Therapy" pages 162-169 for more information. It might also decrease pain too, in addition to helping you evacuate your bowels.

Hope you can work with your doctor to find something that helps