There is only a single controller output going to both trailer brakes. Typical failures are wiring to the brake on one side (left hand in your case), or bearing grease got into the drum severely reducing the braking power. In any case it sounds like it's time to remove the drums on both sides and see what's going on.
To your point that you are not a mechanic, this is an issue that you should have addressed by someone who is. The controller (as noted by Grumpy_G) is only masking what is a problem with the brakes. Brakes require occasional adjustment. Typically, this is done during your annual wheel bearing PM. There have been several…