It doesn't sound as though you've been attacking the problem from the rear end. You have to do some type of cleansing, otherwise those critters either live in your colon or die off there and give you die off symptoms. Do either colonics, enemas or salt water flushes. I personally did colonics but if you search this site you will see a lot of people recommending enemas. I also did a couple of salt water flushes that gave me some results. I will be doing enemas on my next full moon cycle.
My doctor recommended olive oil or coconut oil on the anus for burning. I had it from my herbal treatment for a while.
I highly recommend that you get a bottle of cod liver oil capsules and freeze them. Once frozen, take 5-6 to start with on an empty stomach first thing in the morning. You can work you way up to about 10-12 capsules/day. Every time I did that protocol (along with an herbal formula and diatomaceous earth) during a full moon cycle, I got critters. You can also use frozen pumpkin seed oil or frozen caster oil. Cheap and easy. Some people just take caster oil straight. I haven't tried that yet. I hear the taste is bad.
parasites create bio films to protect themselves. You have to destroy that bio film. I used bromelain, (which is pineapple enzymes) between meals. It made a huge difference. Other people use chlorine dioxide, also known as Miracle-Mineral-Supplement .
You have to attack these critters with everything you can. Beets and beet juice worked as adjuncts to herbal therapy for me. Everyone is different. A couple of other resources are Dr. Andreas Kalcker's protocol- avail online and the book Healing the Symptoms Known as Autism by Kerri Rivera .