Calculating Cumulative sum for last year

Well, you have to shift the dates one year back. So, something like:

CALCULATETABLE(
    // This will move the entire
    // selected period one year back.
    DATEADD(
        'Date'[DateTime],
        -1,
        YEAR
    ),
    VAR MaxDate =
        MAX( 'Date'[DateTime] )
    VAR CurrentSelection =
        FILTER(
            ALLSELECTED( 'Date' ),
            'Date'[DateTime] <= MaxDate
        )
    RETURN
        CurrentSelection
)

Put this as the filter under your CALCULATE instead of your FILTER. This will be the running total for the same months but 1 year back. If you want to get the sum for all the selected months, then change your FILTER to be:

CALCULATETABLE(
    // This will move the entire
    // selected period one year back.
    DATEADD(
        'Date'[DateTime],
        -1,
        YEAR
    ),
    ALLSELECTED( 'Date' )
)