I am far from an expert but I gave it a shot and think I was able to accomplish what you are looking for. The attached workbook gives totals for current and previous month (in relation to a Month parameter) then calculates the difference between these two totals.
To do this I first changed your Month Parameter to a Date/Time value to simplify calculations. I then created two calculated fields, # Last Month and # This Month that handle inclusion logic using DATEDIFF('month', [OpenDate], [Month]).
Hope this helps.