Practice on the lunge line with someone. Someone will lunge your horse as you go at trot on the wrong diagonal and switch to the correct diagonal. You’ll see how unbalanced your horse is on the wrong one. If you practice this start going it without looking and you should get the hang of it soon! Hope this helps!
first looking at his feet*
You need to feel your seat, you can try to sit two strides and change diagonal, first looking at your feet and then without looking, so that your brain assimilates what is the sensation of going on the diagonal😊