If you have plenty of airflow, adding a window unit really won't help to much. It will just use up more electricity and add to your electric bill. Western exposure is the hot side, not southern. Closing curtains or shades cuts out A LOT of (window) heat. Insulating the wall between the garage and your sewing room would help a ton! Yes expensive, but will really pay off in the long run. Also change to florescent light bulbs will cut down on the heat that comes from lights. I'm sure most of your heat is coming from the windows and the garage attic.