You might already know this; to reduce/stop trailer sway use the manual lever gently on the brake controller while maintaining speed. This pulls the trailer back in line with the tow vehicle, thus stopping sway.
One cause of trailer sway is trailer axle mis-alignment; it's easy to check this yourself and the instructions are on the web.
Another cause is having P-rated tires on the tow vehicle, they have weak sidewalls and induce sway, especially when not inflated up to the Federal Placard on the door post or tire sidewall.
This is the
best instructions for properly setting up a WDH I've ever read or seen.