First, this is a good reason to never book directly with the cruise line. They represent themselves, not you. You should always book with an agent, because they represent you and will do whatever they can to support you. They also have contacts and tricks not available to you.
Now, with all that said, since the final payment date on your cruise has past, Carnival will not allow you to change your pricing. They established these new policies about 2 years ago. The only time you would be allowed to change it is if you booked it under the Early Saver Fare.
However, even though they cannot get you a cash refund, occasionally your agent can get you onboard credit or an upgrade on your cabin. This is where it also pays to have a good agent because if you ask Carnival to do this for you, unless you're a past guest with alot of cruises, they're going to tell you there's nothing they can do for you.
So, unfortunately, what your PVP at Carnival told you is the truth - there's nothing they will do for you.
But, what I suggest you do is call Carnival, ask to speak to a supervisor, and then ask them if they can at least get you an upgrade. There are still plenty of cabins available on that sailing and it doesn't cost them anything to provide you an upgrade. It's not much, but it beats a sharp stick in the eye.