You can put a kind of collar around the neck oh the horse and hold it during the course. It will oblige you to keep your hands in the right place and they won't get too long.
By saying "You can hold each rein in each hand" I think Kareen means holding both reins in both hands!
T. a. Eyo is right, as your trainer to correct you as often as possible. It must not be pleasant but it is a blessing in disguise!
I also haven't dealt with this problem, but I do have some ideas.
You could try breaking you courses down into elements and be conscious about letting your hands follow you horse's mouth. Similarly, you can pick a place on the neck to keep your hands and shorten your reins to the appropriate length to accommodate that location. Only issue with the latter method is that it might lead to stiff hands.
It might just be something that you need to be reminded of as you're going around. Ask your trainer if they can yell at you :P In the past, I've benefitted from having things repeated to me over and over again until I got it.
I do not have this problem so I cannot say if the techniques I am going to tell you really work but it costs nothing to try.
You can put some Scotch tape on the reins and "fix" your hands on the Scotch tape.
You can hold each rein in each hand.
You can put a piece of Scotch color tape in the place your hands have to be, so you have a landmark.