I work with a lot of business owners who are working with a professional designer for the first time, and it's normal to be a little freaked out sometimes. But you might find this helpful:
You will occasionally disagree. Give your designer detailed feedback, but do also listen to their reasoning and at least consider it. Mention the parts that are really important to you, and usually your designer will develop a solution that makes everyone happy. (Well, at least I do. :p )
Your designer, if he or she is really good, will probably have a LOT of questions for you about your brand. It may seem excessive, but keep in mind that the more thorough they are in the research phase, the more accurate to hitting the mark they get.
Finally, remember that you guys ultimately are a team. You're a business owner, and as a freelance designer so am I. I absolutely understand how this process can sometimes be simultaneously exhillerating and maddening. And it benefits my business for your business to be successful — I absolutely want what's best for your company, not what looks the coolest in my portfolio.
And at the end of the day, your not being happy is NOT an option. I will not allow you to not love what you get. :^)