All the references that I have checked suggest a MUCH lower dose of grapefruit seed extract...at most 15 drops 5 times a day for a therapeutic effect and not as a maintenance dose. I don't know if this could pose a problem for you or not...something to consider.
Also, the problems you describe after eating are not only associated with parasites...they can result from many dysfuncitons of the digestive system. Since Grapefruit-Seed-Extract is extremely bitter, and since bitters can improve digestion...I wonder if that is why the Grapefruit-Seed-Extract helps with your symptoms.
Cutting out all dietary fat is not the answer and can produce some significant problems...you need essential fatty acids and several vitamins are fat-soluble.
Address your constipation and liver congestion and your digestive system should improve.