Yes, you really DO need a short sale certified agent, who has completed multiple short sales. There is tons of additional paperwork involved that is specific to a short sale so a regular realtor would not have this background. Plus, it will take months to get an approved offer through your bank, and a short sale agent is accustomed to that.
If you do not have a contract with this agent (even a verbal one??), then simply explain the situation to her and apologize that you wasted her time. Paying her even a small amount could indicate that you have a contract and she or her partner could then expect a full commission when you do eventually offload the house. As a result, be honest. Tell her that you didn't realize you needed a short sale agent. That you appreciate her time. And that you will try to make it up to her by referring other clients, or something like that.
Finally, no, you absolutely do not need a third party to negotiate with your bank. A short sale agent can do all of that for you, or advise you what you need to do to communicate with your bank. Are you certain you qualify? Have you already stopped paying your mortgage? Many banks won't approve you for a short sale until you're three - six months behind, and they'll need proof that you can't afford the house.
Good luck. Short sales are not easy, and they negatively affect your credit almost as much as a foreclosure so make sure this is what you want before you jump in. Last but not least, depending on the state you live in, you may have to pay taxes on the amount the bank forgives. For example, if your current mortgage is $200,000 and the offer the bank ends up accepting as a short sale is $150,000... and they agree to forgive $50,000, in some states you have to pay taxes on that $50,000 because it's considered "income". A certified short sale agent should be able to tell you all about the laws and requirements in your state.