There are different ways to do what you're doing, but given the description of where you are (right after filling a circular selection), I'd Select - None (Control-N) then Filter - Blur - Gaussian Blur and set the Radius to taste.
Can you tell me another way to do this using a keystroke, if possible? I only remember that it was something really simple and a stupid little detail I overlooked. I tried the method above but I can't get it to work in my version of Photoshop. Thanks in advance.
None come to mind, except perhaps if you created an action to do the steps I decribed and assigned it to a function key.
Once you do a Gaussian Blur one time, and assuming you don't do any other filters, you can do the same Gaussian Blur filter again with the same parameters by pressing Control-F. That works with most things in the Filter menu. Basically Control-F is a "do it again" function.
Maybe someone else with a better crystal ball can come up with a different idea.