You were referred back to the agent because that is ALL cruise lines policies, if you book with an agent, they are your AGENT designated to act on your behalf with the cruise line, if you book directly with the cruise line, then your cruise line representative in effect becomes your agent they will work with you. You are in a bummer deal at this point but DON'T let it turn you off to cruising OR to using an independent agent or agency, they are not all like the one you have now. BUT, next time around, make sure you discuss (a) cancellation fees, (b) change fees, this includes upgrades, (c) rebates if fares for your cruise/cabin category go down. If they want to charge you for any of these or if they will not pass on price reductions, move on. Finding a good agent that you are comfortable with is a hunt an peck process, takes time, energy, and perseverance but in the end it is worth it. Keep in mind in the agent vs. cruise line representative dilemma, a good agent works for you, a cruise line agent works for the cruise line, do the math.
Suck it in, bite the bullet, chalk it up as a learning experience, just unfortunate that you had to get caught in this on such a great destination, but remember the worst cruise in the world is better than the best day at work, so spend more time on deck, in the public areas and soak in the beauty of Alaska.............
Oh and make sure you convert some money into Alaskan dollars before you go...