When someone is labeled as a "sight-reader," that usually means that he has not learned how to decode or "sound out" words, but instead learns them by memorizing them as whole units. As a result, it becomes very difficult to read challenging text or to unlock the reading of words you have never seen before. Spelling is also affected. This is a learning disability that can be identified through a comprehensive evaluation and can be treated successfully.
There are several excellent programs that can help adults with this reading disability to become good readers. My favorite is the Wilson Reading System. You can find out more about it by going to the Wilson website at http://www.wilsonreading.com. In the meantime, you may be eligible for getting textbooks and other books on tape. Go to http://www.rfbd.org for more information.