Constipation is defined as lack of a bowel movement in more than 72 hours. About 1-6% of healthy adults suffer from constipation. Constipation can be due to many reasons generally subdivided into several groups:
I. LIFE STYLE:
1. Lack of physical activity;
2. Not enough fiber intake with the food;
3. Not enough liquid intake;
4. Ignoring of the normal urges for defecation.

II. MEDICINES: verapamil, opiate painkillers, neuroleptics, antidepressants, drugs for Parkinson, iron supplements, antacids, long term used laxatives, diuretics etc.;

III. ORGANIC REASONS: anal fissure, intestinal stenosis, tumors, endocrine disorders (hypothyroidism, hypoparathyroidism, hyperparathyroidism, Addison’s disease, panhypopituitarism), diabetic neuropathy, neurological disorders (Parkinson, multiple sclerosis, cerebrovascular disorders, spine cord injury), and

IV. PSYCHIATRIC DISORDERS: anorexia nervosa, depression, etc.

The most important thing in evaluating constipation is to exclude organic reasons (tumor, ileus) for constipation.It seems likely that you have already made all the necessary medical tests (colonoscopy, intestinal passage, manual rectal examination, occult blood test, hormonal analyses etc.) in order to exclude organic constipation. Long term constipation is usually not organic.
Chronic constipation is treated with:
1. Life style changes: increased intake of fibers and liquid, increased physical activity;
2. Abortion of stimulating laxatives;
3. Avoiding medicines that cause constipation;
4. Using laxatives that increase the stool volume: osmotic laxatives, lactulose, magnesium milk, natrium picosulfate.
5. Enemas.

You may want to consult a gastroenterologist about your problem.