I was able to solve this by finding the total amount of rows in your field by using get_field_object
.
In your code, after if( have_rows('cm_content') ):
try this:
$field_object = get_field_object('cm_content');
$total_rows = count($field_object['value']);
Then you can test for it with an if statement:
if($counter == $total_rows ){
// this is the last one!
}
Cheers,
Andre