1. Can anyone tell me what I've done wrong? Like, am I modelling things completely wrong.
2. Can you tweak my code so that is works?

I'm trying to have a Filter use the current value being evaluated in a parent SUMX. In pseudo code, I'm trying to do: (The ith value is the issue)

Sumx(  
      myDistinctValues  
      , Calculate(  
              myNumber  
              , Filter(  
                    myFacts  
                    , [myCat] =   
               )  
       )  
)

I want to iterate over each row in myDistinctValues, use that value to Filter myFacts, and then return the sum. However, inside the Filter, it returns all rows; so my Measure throws an error.

My current DAX looks like this:

this_Measure = 
    var myFacts =
        AddColumns(
           
           ,"myCat"
           , 
         )

    var myDistinctValues = 
        Distinct(
           selectcolumns(
               myFacts
               , [myCat]
           )
        )

return

   Sumx(  
      myDistinctValues  
      , Calculate(  
              myNumber  
              , Filter(  
                    myFacts  
                    , [myCat] = myDistinctValues
               )  
       )  
   )