Hi Edward! To get the YTD value while only showing the last month, you should set up your sheet as you posted (with all outstanding months in the year filtered in) and then use a table calculation filter to hide all but the last month.
It's quite simple, luckily. Just create a calculated field like so:
LAST() = 0
Put that in your filters shelf, make sure it calculates along Date (or Table Across, in your posted example), and then filter in only TRUE.