Your insurance through Carnival is not refundable. If it were, everyone would buy the insurance and then cancel it right before sailing, when they knew they'd be able to go.
You can still buy outside insurance in addition to Carnival's insurance, but it's probably not worth it at this point. There may be higher coverage amounts, but you're right that you would not be covered for pre-existing medical conditions. And I think we can assume that Carnival will not go out of business between now and a month from now, so the insurance will still be valid!
For the next time, there is better coverage out there, including things like "Cancel for Any Reason" insurance and "Cancel for Work Reasons" insurance, through outside companies. Hopefully, you won't need to make any claims on the insurance at all this time, and you won't have lost anything.
Have a great cruise.