Before trying to see the diagonal (I wouldn't bother trying to feel it yet), focus on the two-beat rhythm of the trot for a couple of minutes. Count it out to yourself, one-two, one-two, one-two, and make sure the tempo is consistent. One-two, one-two, not onnnnnneee-twwwoooo, onetwo. When your horse is trotting in a nice energetic but relaxed, rhythmic tempo, start watching the outside shoulder. It goes forward, back, forward, back. Where is it when you sit down? It should be back. Don't try to watch every single stride--just check 3 or 4 times around a circle. Also, don't *stare* at the shoulder, just glance at it out of the corner of your eye. If you stare at it, you'll lose your focus on the up-down, one-two rhythm.
Once you've gotten so you can identify the diagonal easily all the time, then you can experiment with feeling it. But this isn't something that you desperately need to ride well. Lots of very good riders quickly check their diagonals by sight.
On most horses I can feel when I'm on the wrong diagonal. I can't explain how but first of all its important that you can identify it by sight. Then once you know how to change diagonals etc, you can really focus on what the wrong diagonal feels like and BAM! There you go :)
It's also a case of practice, sometimes you don't even realize that you can feel the right and wrong diagonal. Trying sitting trot and try and feel the kind of dips as the horse's shoulders rise and fall. Just keep practicing and like other have said get used to the rhythm ^^
First you need to be pretty good at checking your diagonals just by looking down. Learn your horse's footfalls (which legs go up/down and when) and start to feel where his hind legs are. When your horse's outside leg goes up, his inside hind does as well. So you can feel it by feeling when his inside hind goes up.
Try what others have said in regards to sitting to the trot and counting beats, this is a good start. Then jump off and get someone else to ride your horse (even better if you can get someone who knows their diagonals). Watch them for a while, decide for yourself which diagonal they are on and get them to swap occasionally so you can see the difference.
Then change and get them to watch you. Try using an active trot as you should be able to see your horse's hoof extend just beyond the shoulder on each side, this may be easier for you to recognize while you are still learning.
And don't worry, EVERYONE has trouble with diagonals at the start but once you get it, it will become automatic!
Try feeling the horses beat. I have the same issue. So what coach says is glance at the horses outside leg which is on the wall. You go up as that leg goes forward. The other leg needs to be forward when you sit down.
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