yes tinidazole is a wonderful medicie, but non of the big books on parasitology suggest it to be used mono therapy against Amebiasis (Metronadinazole or Tinidazole should not be used mono against any Amebiasis). This will more often than not often lead to no cure.
If your doctor do not know this, he probably do not even know how to properly test for Amebiasis (E. Histolytica) and also other parasites....
20 years of natural therapies has not worked for you to give you a cure....and yes seems like your doctors not know what they are doing...
Find one of the few doctors in world that actually follow the books, that know how to diagnose and how to treat. Then soon likely you also cured, like a series of us already are (after also going down a lot of dead end roads...)