Health insurance issues. I have health insurance but I have to switch, coming off my parents but.. question.?

I have a "preexisting condition". I have a benign(i.e not bad) nodule that developed on my thyroid. The only recommendation I have is further testing twice a year. Every health insurance company Ive gone to seems to approve me for insurance but only with the added point that I will be ineligible for coverage for life for absolutely anything that has to do with my thyroid and or any disease that may originate from me having a problem with my thyroid. This all apparently due to the fact I have one nodule that is not even malign but benign. As the thyroid is a very important part of the human body and heavilly interconnected with other body structures/functions I am basically excluded a serious % of my coverage for life. Can anyone explain clearly why this happens to me even though I have been continuosly health insured but with another company? Does anyone have any ideeas of a health insurance plan/company who will not do this to me when approving me?