Since you don't get the expected relief from the dietary changes and consumption of water in large amounts, you need to be evaluated for underlying cause for your constipation.
There are many causes for this problem, like use of certain drugs that can lead to constipation as their side effect (antidepressant, antihypertensive drugs, narcotic pain killers, iron supplements, antacids), prolonged abuse of laxatives, hormonal disturbances like conditions with either too little thyroid hormone (hypothyroidism) or too much parathyroid hormone, diabetes, scleroderma (conditions that affect body in general and have adverse effect of gut, too), rectocele in women, diseases of brain and spinal cord affecting nerves and muscles function of gut (Parkinson's disease, multiplex sclerosis, spinal cord injuries).
Also, you may have a constipation predominant form of irritable bowel syndrome, a functional disorder of gastrointestinal tract.
Having a chronic and severe form of constipation puts you on risk for fecal impaction (a large mass of dry, hard stool that can develop in the rectum).

Visit your physician and start with simple blood test, blood test for hormonal status, and rectal examination.
There is a possibility for you to need to be checked by gastroenterologist with more advanced techniques!

Best wishes!
Marija