Lost control when towing?

The scariest situation I had was when the trailer started to sway wildly. I was using a flatbed to haul my pick-up that had broken down. I was going to borrow a friends dually but it wouldn't start so he loaned me his other truck, a 1500. I had to drive my broken down truck on the flatbed backwards because only reverse worked. This made for very little tongue weight.
So I had about 7,000 pounds behind a 1500 with low tongue weight. Sway went crazy at about 45 MPH. Slowing down and lots of steering kept it from flipping.