For approximately the first 4 or 5 years of my riding, I could NOT pick up the right diagonal. They would say "follow the outside leg!" and I would try to, but it simply wasn't working. When I went to new barns they would put me with beginners who couldn't canter when I was already jumping just because of the stupid diagonal! Lo and behold, one day while once again frustrated by the diagonal...I checked the inside leg instead of the outside leg. I saw myself moving against the leg in the opposite pattern of motion to the horse's shoulder, and suddenly I got it. Now (2 or 3 years later) I really just have to check really quickly on either shoulder if I'm unsure
I am starting to be able to pick up diagonals without looking, and I can still tell when it feels "off." I can't really work on it much anymore because my lease really likes counterbending on straight sides so he can look around at what's going on around him (not around circles and turns thank god) and it makes it difficult to feel. You'll get it eventually!