I'm trying to remove the valve stem in the shower (hot water) and I can't get the stem to come out. The piece that has the flat sides (only 2) I believe is supposed to unscrew and it won't budge. I would like to avoid breaking it but it won't budge. What can I try?

I have placed a image of a three handled tub/shower valve,(see image) for you to go by. Most valves are set up this way. Unscrew the nut closet to the valve body and back the stem and washer assembly out counterclockwise. Good luck, Tom