I have a healthy diet: plenty of fruits, vegetables and meats; I drink a lot of water; my grain and diary intake is low; and I am active. I am consistently constipated. What can I do?

Add fiber foods. Add beans, seeds, whole grains. Got celiac sprue? Add gluten free whole grains. Pro biotic containing foods may help ( fermented), e.g. Tempeh, miso, fermented dairy, like yogurt, kefir.
Heal anal fissure. Try zinc oxide paste. Stay away from dairy, wheat and rice entirely. Quinoa is a good grain. Use baby p spinach leaves, pitted prunes, and canned coconut milk as a cream in warm drinks. Eat your water with more vegetables. Eat 6 equal meals follow by 5 minutes relaxation breathing. See kelly mcgonigal the will power instinct, grab 5 positive see rich hansen. Lowering stress may help.
OTC. There are numerous otc products containing fiber materials designed to bulk up stools and create a regular bowel schedule. Try these. See your doctor if this doesn't work. You may want to have your thyroid function checked. Good luck.