You also need to explain this statement from your first post more fully.
The table is sorted automatically by access
It seems you are talking about the sorting in the table being scrambled, not in the combo box.
My instinct tells me that you might have the field storing your month/year string set as a key field and this might explain the alpha numeric order of the records in the table, whereas you want the records to remain in the order they were entered.
Is that correct?
I don't have a full understanding of how it works in access (my DB engine of choice nowadays is MS SQL Server) but when you make a field in a table a key field an index is automatically created for that field to help make queries run faster when you use that field in a WHERE clause.
One type of index causes the records in a table to automatically be sorted alphanumerically like in a dictionary. It is that sorting that provides the speed improvement when querying that table.
I am not sure I am on the right track for your problem here as you don't give us much detail to go on
And limra is correct, if you want dates to sort as dates then you probably should save them as dates and not strings.
You could also save them as numbers in YYYYMM format