Bridesmaid wants to be maid of honor?

I just got engaged and am planning a wedding for next spring. I have already chosen my bridesmaids and maid of honor but haven't announced it to anyone accept a few family members. One of my bridesmaids is being very pushy about being my maid of honor even though I have already chosen somebody else. I have known this girl for many years, even more than my maid of honor so I do not want to hurt her feelings by telling her she isn't going to be my maid of honor. What is the best way to go about doing this?