Calculate monthly average in pivot table

Hi there!

I am attempting to do something that I'm sure is really simple, but I just can't figure it out.
I have a table of sales data (the source is a power query)... and have created several pivot tables from these data.
One of these pivot tables shows each service in Rows, and transaction dates (grouped by months and years) in Columns. The Values are sum of qty of sales (units) for each service in each month.
Table.png

When I change the Value Field Settings / Summarize value fields by to Average, the pivot table calculates the average of the units within each month rather than the average per month (across all the months):
Current average.png

What I would like to do is to add an Average column (ideally in the same pivot table as Sum of Units/Total) that contains the average monthly sales of each service across the whole time scale.
Needed.png

I would really appreciate any help in figuring this out... I'm sure I've got a facepalm incoming!

Thanks in advance!

Greetings,

I am trying to replicate the solution above, but making an amendment to the average calculation by including a distinct count of the total number of months that have passed + the current month. The implication is that the denominator is not biased upwards unnecessarily making the average smaller than it should be.

I followed the steps exactly as the solution posted, but I still run into an error.

1. I added a field, call it Sum_Expenses, which has the following formula: =Sum(Expenses_Amount) where Expenses_Amount contains all the expenses I am summing.

2. I tried to add another field that averages across months, call it Average_Expenses, which has this formula: =[Sum_Expenses]/SUM(IF(ISTEXT('Expense Data'!E2),1/COUNTIF('Expense Data'!E2:E9999,'Expense Data'E2:E9999),"")) where the denominator counts the number of distinct months (text variable) in a year. The numerator and denominator each work fine independently, but when I try to add this field, I get this error (see image attached).

Note that I used E2:E9999 because for some reason $E:$E doesn't work. Presumably I can run a slicer on the year and the average will adjust accordingly.