Future Mother-in-law Driving Me Insane!!?

My future mother-in-law has me stricken with panic attacks almost every time I see or speak to her lately. When we first got engaged, she was so ecstatic and wanted to start wedding planning immediately. She would tell me what "she" liked what "she" doesn't like, the type of dress she would like to see me in, colors, everything. When we met with vendors she would just talk and talk and never listen to anything I or other people would say. She is the least focused and one of the most child-like women I have EVER met. She speaks without thinking and says things without taking responsibility for them. With my mother in a different city, she has just been driving me nuts. Here the BIG dilemma. She offered to pay for the alcohol and flowers. I know that you can never expect anyone to pay for anything unless they offer...but SHE offered. Vendors have been hired and the budget has been created in accordance to her offer...now she is reneging. She nonchalantly told me today that her friend said that the grooms parents don't pay for those things....but she ALREADY offered!!!! I am so upset with her and her disregard for common courtesy. I already called my fiance and he said he would speak with her, but I don't want to cause conflict between she and I, nor do I want a marriage with a mother-in-law who feels negative towards me...WHAT DO I DO? How do I handle this type of woman and how do I handle this wedding paying issue?!?