You applied very late for your visa. The normal processing time for any Schengen visa is 3 weeks. You should apply at least one month before you require the visa or even earlier than that.
This is your own fault. All you can do is call the Schengen office and try to ask them to expedite your visa
You will not be given a multiple entry visa if you give the details of only one trip. You must provide information of all travels proving as to why you need the multiple entry visa.
You cant just write a letter and say "oh by the way we will be travelling again in December"
Thats why you were refused.
You may have difficulty now in obtaining another visa.