A colon cleanse is going to do nothing to help you with this problem. If you're having problems at the lower esophageal sphincter you need to stay on the nexium, or find another PPI that seems to do a better job - not each med works well for each person. It's trial and error. In addition, you should be following a GERD-friendly diet. You can find a representative example at gicare.com.
Have you been tested for H-phylori? as that will cause these symtoms you have. The constipation is most likely a different health concern so try taking magnesium citrate with calcium making sure magnesium is higher than calcium levels. Take 250-1000mg of magnesium with each meal and increase or decrease to adjust for your bowels. Calcium and magnesium go together yet Calcium in some with cause constipation and magnesium will help relax the bowels and increase bowel movements.
Natural remedies for heart burn, as nexium is only a bandade solution, calcium citrate, greens drinks, chlorophyll, and baking soda 1/2 tsp 3 x day. If you are sensitive to sodium then take no salt as well as the baking soda.