Put one hand on the bottom of the steering wheel. Push that hand in the direction that you want the trailer to go.
Something else to try when straight backing, if you are having a hard time staying straight. Look past the end of your trailer to see if what you can see is changing. You still want to pay attention to the other stuff around you and the pavement, but if you are straight backing in daylight, there is almost always something you can see in your mirrors past the edge of the trailer, on either side. If what you can see changes, you are no longer backing straight.
That comes in handy when you have to back nearly all the way down the south side of Tiger Mountain Hill in the dark and rain due to a major truck crash.
I have had to make a couple really long backs in the dark too, but never down a mountain in the rain at night. The worst long back I have had to deal with was a mile-long back I had to make after being misdirected by a road construction crew onto a state park access road rather than the unmarked detour. I was more than a little unhappy when I saw the locked gate. Trees on both sides of the road. Light posts every quarter mile. Took me over an hour to get back out.
The best advice that I can give you is that backing in requires two things to be successful. The first is the setup. If you are not set up in the right position with the trailer it is very difficult to back in. The second is the follow-up. I'm sure the YouTube videos and a toy tractor trailer would help as it's very difficult to explain this by writing. But if you rememberi it"s just two steps you need to understand each one. And if you need to pull up and get further to the right remember that if you pull up onto a 45 degree angle, however hard you turn left points the trailer further to the right. That is the setup. Then for the second part to follow up you have to not follow up as sharply and the trailer will move to your right. If you can keep it simple and think of it as two steps I think it will work out better for you if you can understand it that way and then it's just a matter of practice. Good luck
That reminded me. The land you are on, if there is ANY SLOPE on it that trailer when loaded WANTS TO GO DOWNSLOPE. Following gravity. You will have to add enough opposite wheel to keep that trailer straight. I refuse to try and convert to english what I mean by opposite wheel against the trailer wanting to slide downslope. I don't have the adequate words to explain it.
Keep calm and dont give up. It will click in your head eventually. Studying with toy trucks helped me ALOT Good luck
Turn head backwards. Turn wheel to the right. Trailer moves right. Turn wheel left. Trailer moves left.