Why not just do a search for each month and change all to the desired abbreviation? It will take care of that in one click. You'll do just 12 finds, shouldn't take a minute. Hope this helps.
I want it in one click for all 12 months....it is doable so why do it the
slow laborious repetitive way?
You can still do it in one click using a 12-part "or" statement with positive look-behind for each month's abbreviation part, then replace with your period and space. It's exactly what you are doing now, but you explicitly list Jan, Feb, etc in your look-behinds.
Actually, you'll need fewer than 12 -- May doesn't need an abbreviation, and you've already left out April, June and July as well.
An alternative would be to search for each word specifically and then use one of the many methods in the following article to daisy-chain them together.