How to get prior version data in a measure

hi all

seeking some help please. i run a powerbi report that takes in demand forecast in a lot of detail, but ultimately the report builds up a profile of demand forecast by month. See below where the columns represent the published forecast month, and the rows represent the demand months

now whilst this measure works, as you can see when we start going close to the end of the calendar year numbers start disappearing. this is because the raw data is rolling each month so the historical month column drops off as a new month comes on. 

i'm trying to create a measure that looks at the latest forecast version where there is data (i.e. in the above case, November's data) but where there is data missing look at the prior version's info. struggling with the dax code here. below was my first attempt but as you can see it doesnt yield the right output (as i believe i'm filtering the data with the TRUE operator)

below is what the data table looks like. each new forecast version gets appended to the dataset.

any help here would be appreciated.