Get entry index - structure not channel
I found the clue for how to do this in the getPrevSibling documentation:


  getPrevSibling() will return the previous sibling whether or not it’s enabled. If you want to get the closest enabled sibling, you can do this instead:
  
  {% set prevSibling = craft.entries.positionedBefore(entry).order('lft desc').first() %}


So this query will get the number of enabled previous siblings:

{% set prevSibsCount = craft.entries.positionedBefore(entry).total() %}


And then your position will be that + 1:

{% set position = prevSibsCount + 1 %}
{{ position }}