The way I handle this is to write a formula that sums "vertically", through the workbook. If you think of a normal Excel formula as being in 2D, this makes it 3D.
To do this you would need to use a consistent cell to store your info. If you have a worksheet for each month, pick one cell for your monthly total and keep it the same for every month. Let's say we choose cell C14.
Here's the formula you need to calculate YTD for December:
I'm assuming a couple of things here.
This assumes your sheets are named Jan, Feb, Mar, Apr etc, and also that they are in that order within the workbook (ie the order is Jan, Feb, Mar, Apr etc).
For each worksheet you'll need to tweak the formula, ie: