I agree with the other broker that not all mortgage companies state you have to be behind in payments to do a short sale. As an example, if you have an FHA loan you need to be behind one month, but that can happen right before closing. It is possible that the bank suggested that as a parameter to your agent.
I absolutely don't agree with the other broker that paying a "company" to do your short sale is best. I have done a lot and I'm a realtor. Those other companies sometimes don't "care" as much about you as a realtor. We work to please our clients and ultimately hope for repeat business. These companies aren't looking to build a relationship with you or repeat business. I do short sales every day and my shoe salesman works for one of those companies. You should stick with your realtor.

Christine- A couple things...
First off in Colorado an agent could be in a little trouble with the real estate commission for saying that to you.
As far as your lender doing a short sale, it is completly up to them, and what policies they have. My guess is your mortgage company told your agent they wouldnt consider a short sale unless you were behind, and that is the inforamtion your agent passed back to you. Real Estate agents are not always the best peson to do the short sale part. Lawyers, other agencies, and companies are out there that can help you, but they cost money upfront for their services, BUT this is what they do everyday. you would like some names give me a call and I would be happy to get you a few. Thanks and good luck!

We're having trouble paying our mortgage due to financial hardships and need to move out, but because of the market we won't get what we owe if we try to sell our house outright, which is why we're thinking about a short sale.

Maybe just keep the house and pay your mortgage-- then you don't need to worry about a short sale. Good Luck