Sometimes approaching things backwards works as well - look for the shoulder that is very clearly BACK as opposed to forward, and line up your diagonals accordingly. Sometimes it's easier to detect the back shoulder as opposed to the forward one.
While riding the Clyde in my avatar photo his mane is so unbelievably long that it literally covers his right shoulder most of the time, so trying to see the diagonal when on left reign was tough. I got into the habit of just looking at his left shoulder instead and getting my diagonals that way.
It all comes with time.
Yes! This! I struggled with diagonals for YEARS...Finally, one day I decided to follow the inside leg. For whatever reason, it "clicked" to rise when the inside leg was back, and sit when the leg was forward. Something about posting to the opposite of the motion really exaggerated what I was supposed to be seeing. To this day, I glance at the inside leg. Diagonal struggles are a common problem!
"The wise man thinks he knows nothing.
The fool thinks he knows everything."