Well, my 10 yr old daughter is going through this. She knows how to post, and knows/understands diagonals. She doesnt always pick up the correct one and her instructor tells her simply "change your diagonal" so she sits 2 beats and picks it up again. It seems to me the more she thinks about it the more she misses it...lol
Her instructor put her on the lunge line several times and would say 1,2,1,2 or up/down
Also: double posting seems to have helped her with the up/down movement and using the right muscle memory.
We also watched a video on youtube about diagnols for her to see the difference and I also video taped her trotting so she could see herself as she rode.
She gets it about 90 percent of the time now, she is still working on perfecting it. She used to ride in a western saddle ,now she says it is hard for her to sit the trot after learning to post.
Horses and children, I often think, have a lot of the good sense there is in the world.
Josephine Demott Robinson Feed, muck, groom, ride. Repeat daily!