Have you read The Body Ecology Diet by Donna Gates or Breaking the Vicious Cycle by Elaine Gottschall? Both of those might help you. It sounds like you definitely have a challenged digestive system and a careful diet, especially avoiding most grains and sugars would be helpful. I have similar issues, but not quite as bad as you. It is frustrating to follow the diets when that's so different than the way the normal population eats, but many people have healed themselves that way. My husband has started low-carbing since he has warning signs for heart disease and diabetes so that's helping me stay more focused on my diet.
Donna Gates does address the subject of parasites in her book. She says that a weak digestive system encourages them to become established and she does recommend parasite cleansing products as part of the healing process.