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it's going to be at least 25 years until I retire
Assuming you don't plan on accessing this money before you retire, you really don't need to worry that much. Unless you think that our economy is permanently destroyed and stocks will remain permanently depressed for over 25 years you have plenty of time.
Look at it this way. My 401k has basically been cut in half too. But I am maintaining my regular contributions at the same level and now my contributions buy twice as many shares of each index fund I own. I am not afraid to do this because I have 30 years before I retire.
When the economy does eventually recover and stocks start going up again -- which could be long time, but I think will be in much less than 30 years -- I'll have a lot more shares to go back up!
Of course, the closer to retirement you get, the more defensive you need to be. But if you truly don't need this money for 25 years, I think you should just hang tight.
(I am not a financial advisor!!!)