Work Sheet Sum Of Cells

This situation is as follows. I have a worksheet with 12 sheets. One sheet per month which is used to show monthly cost expenses for certain categories.
I have a budget category, a paid this month, a paid previous months and paid year to date. I suppose I can take the paid previous months out if I can get this to work but I need to calculate one of those cells (prev months or ytd) to calculate and sum the previous months and then tack on the current month.

Previously I have had these categories and used =SUM(Month:Month!A1)
Like --- > =SUM(Jan:Nov!C10)

This works and all I have to do is add on the current month to paid prev and get a ytd but this also means that I have to rewrite the formuals concurent with the noted month. Is there a way that I can make one sheet as a base and then just make 11 copies with a formula that will sum all previous worksheets to the start at a certain cell reference. So say sum all A1's on all worksheets before the one that is currently in use say for example Dec.

If I need to rewrite them that is find but I figured there should be an easier way to do this so I would not have to rewrite each month individually and then copy the formuals through that sheet.