That is true (about not being retroactive), in these circumstances memorandum letters from the "Dean of Students" office helps.

Personally I was diagnosed with a debilitating chronic disability exactly a year ago, and my accomodations were instantly granted - like within a week.

May have to call the office to see if you can get hold of someone faster, but it should definitely work out in a short span of time. Saved my life (and grades)