How Can Chronic Constipation Be Treated?

I have had constipation on and off for four years. I sometimes take a stimulate laxative or Miralax when it gets very uncomfortable. Some of my symptoms are a fist like feeling to the right of my belly button(this is often after I have eaten) which comes and goes or feels like it moves lower into my rectum. I have to strain a lot when trying to move my bowels. My stomach gurgles a lot after meals and I can sometimes feel my lower colon moving if I put my hand there, particularly on my lower right had side above my groin area near my thigh. Sometimes I feel a lot of pressure and discomfort in my upper abdomen. I have bouts when I pass a lot of gas. I am a fairly active person and usually walk 4 miles or more a day. I religiously drink at least 5 8 oz of water every day and being a vegetarian I have always eaten foods high in fiber. In the past 4 years I have had 2 colonoscopies, both of which were good and there were no polyps. When I was 46 I had a section of my colon removed because a diverticula had grown outside of my colon and had wrapped itself around my left ovary. After 6 months my colon was reconnected and I had no problems for years. In the last 5 years I have felt from time to time as if tissue inside where the reconnection was is tearing away from the stomach wall. There is no pain with this either. Over the past 3+ years I have dieted and lost 35 lbs. I now weight 108 lbs. During this weight loss period I ate a much smaller quantity of food. This is around the time that I started to have the problem with constipation. I now eat a much larger amount of food, but it doesn’t seem to make any difference. So, what are your thoughts?