It does not sound like your controller was turned up too high. It sounds like the brakes are adjusted too tight and are dragging. But presumably you towed it home from where you bought it and didn't have this problem, right? Here is what I would do.
First, with the trailer disconnected from the truck, jack up one side of the trailer and spin the wheels by hand. Properly adjusted brakes, you should just be able to feel and hear them dragging the slightest bit. If they are significantly dragging, then the brakes are adjusted too tight and that is probably your problem. If they don't drag at all, then they are too loose and need to be tightened up a bit.
You should refer to your owner's manual to see how to adjust the brakes. Most of them, there is an oval-shaped rubber plug in the back of the hub that you pry out (I pull it out with a needle nose pliers). Then there is a hole, inside of which there is a star-wheel that you turn. Turn it one way and the brakes get tighter; the other way, they get looser. You can use a screwdriver to turn it, but you will be much happier to get a brake-adjusting tool called a "spoon". It is basically just a piece of metal with a right-angle turn at the end, that allows you to insert into the hole and more easily move the star-wheel. I improvised one myself out of a piece of metal I had laying around the shop. It is MUCH easier to use than a screwdriver. Well worth it, IMO.
Here's a picture of a "professional" brake spoon:
Some trailers have auto-adjusting brakes. The way these work is, you attach the trailer to the tow vehicle and then hit the brakes hard while backing up. I've never had one of these, so I can't say much more about it.
Once you have all four of your brakes adjusted correctly, the next step is to hook up your tow vehicle and plug in the electrical connector. With it hooked up, again, jack up a wheel and see if it spins freely. If there is a malfunction in your wiring, simply hooking up the truck may cause the brakes to actuate.
Finally, have someone press the manual activation lever on the controller and confirm that the brakes lock up and you cannot turn a wheel by hand.
With regard to adjusting the controller's strength, this is a "feel" thing. IMO, if you are locking up your wheels, that's too much. What I pay attention to is, when I hit the brakes, does it feel like the trailer is "pushing" or "pulling" on the truck. If I hit the brakes and I feel the trailer tug on the truck, that means the trailer brakes are too strong. If I hit the brakes and I feel the trailer kind of push on the truck, that means they're too weak. When they are properly adjusted, the truck and trailer both stop as a unit, with no tugging or pushing. This all really only matters at surface street speeds. At highway speeds, the controller is pegged all the way up unless it is very lightly loaded.
