How can I sack my bridesmaid?

Talking to a friend about my wedding and out of the blue she asked if she could be my bridesmaid, I laughed but she was deadly serious and asked why I was laughing. She caught me off guard and I couldn’t think of a nice way of saying no, so said “erm….if you like”.  She’s now planning speeches and looking for readings! She is a good friend but not one of my best. I’ll feel guilty if I take back the offer but resent her for putting me in the position. At the minute I feel I only have the options of either resentfully letting her be bridesmaid or telling her she can’t in which case I know she will fall out with me.