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.