Sum across columns based on column headers or count of columns

I'm writing a macro using Office 365. I'm a beginning/intermediate vba coder. I only get the opportunity every now and then so I feel my way through.

A piece of the macro needs to be as follows:

Workbook Setup:
I have two sheets in a workbook.
One has a pivot table summing the revenue of customers by month date for 1 year. Columns = months, Rows = Customers
The second sheet is a pivot table in the same format, for different. On the second sheet however, there is only a partial years data. So on the first sheet there is Jan-Dec and on the second, only Jan-Mar. The amount number of months on the second sheet changes so it needs to be versatile depending on when the report is run.

What I need them to do:
I need to create a column on the second sheet, at the end of the pivot table, that sums the same months from the first sheet on each row. So, if the second pivot table has months Jan-Apr, I need to the macro to sum across the totals for only Jan-Apr from the first pivot table.

I have attached a workbook that hopefully will make this a little clearer. Any suggestions or code on how to approach this would be very helpful.