A suggestion would be to gather all of the medical documentation which states exactly the time and day in which the final diagnoses was made. Contact the agent who sold you the insurance and discuss this information, always keeping the original documents.
There are two ways of getting help: One is through a lawyer who can directly contact the agency and request a copy of the contract that you both signed, and that way they can demonstrate the breach of contract. Some lawyers will only charge if you get money out of a lawsuit against an insurance company.
The other alternative, which I came to find out about just recently, is that every state has what is called a "department of insurance" or an agency that operates under the same concept of investigating claims and the practices of the insurance company. Search in your state or through law firms within your city to find where that place is and proceed with all your documentation to make things right.
Remember that insurance companies are basically placing bets on our time line when they offer certain benefits assuming that they will not be used or cashed in. They are also private corporations and they can get very greedy. Just check below on the link on that info about the department of insurance, and for lawyers in your area.
And I truly and sincerely hope this is of some good use for you.