i would simplify this a bit by just altering the date of any record you want in the previous category as one month ago today , then you can group on the field by month and alter the header to show as you want.
if {Command.ReqDate} < date(year(currentdate),month(currentdate),1) then dateadd('m',-1,currentdate) else {Command.ReqDate}
use rhis to group on in the CT set to monthly
rigt click on the column header
select Format field,
select common tab
select display string formula
if currentfieldvalue< date(year(currentdate),month(currentdate),1) then 'previous'
else totext(currentfieldvalue,'MMM')