Another question that has come up since I'm preparing for my first show:

No, you're not marked down if you're on the "wrong" diagonal, but there's a reason why the outside diagonal is the preferred one. Using the correct diagonal helps the horse maintain her balance, and if you choose the wrong diagonal, and it makes the horse less balanced, you will get marked down for the wrong diagonal in the collective marks on the rider's position. In the lower levels--especially Intro--the young horse is probably already lacking in balance so being on the wrong diagonal wouldn't make much difference, but you don't want to make it any harder for the horse to find her true balance. If you're using the same diagonal all the time, you will interfere with that learning process for the horse.

It's not the horse, I have this problem no matter which horse I'm on.  For some reason it's nearly impossible for me to catch the right diagonal, and then I'm off balance when I do, making things worse for the horse.  

 Correct, diagonals are not scored in dressage, so it's not a big deal.  If you are having trouble however, I'd guess skeletal or muscle imbalance.  Due to back and leg injuries, I've been working with a chiro and PT a LOT this year, and it is AMAZING the difference it makes.  Finding and strengthening your weak areas can really change your riding and I am slowly becoming able to do things that I thought just were impossible for me before.

Maybe a few lessons from an instructor could help you learn how to pick up the correct diagonal on the right rein. Even if it's your imbalance--as Solaris suggested--an instructor should be able to help you find a coping mechanism.

I do take lessons as often as I can.  I think my instructor is ready to kill me!  She did tell me that people who start riding as adults sometimes have this issue, having one side stronger than the other.  I've always been able to sort out my other issues,  I eventually have one of those "aha" moments when I figure it out.  For some reason this simple little thing eludes me.  We've tried several different things, I always end up where I was.  It would be funny if I wasn't worried about ending up with an unbalanced horse.  I do have back problems, so I'll try some stretching/toning and see if it helps.  I also have a tendency to get tense as I ride.  I plan on going to the round pen today and working on nothing else to try to get this sorted.  I can't stand not getting it right.

Try riding down the centerline, where you are not really on one rein or the other. Practice changing diagonals every five strides, and do not look down, just do it. Once you get the hang of doing either diagonal, go back to the track on the left rein, but don't post on the correct diagonal, post on the diagonal you'd use for the right rein. You'll get the rein you prefer--the left--with the diagonal you dislike, the right. You will find--I suspect--that it's all in your head. The left diagonal you like is just the same as the right diagonal that you dislike. That's what you'll learn by changing diagonals on the centerline. I'd bet not looking down to check your diagonal will help, too. Ask a friend to watch you and tell you when the horse's correct front leg is going up or down, and eventually you will not need to look to get the correct diagonal, you'll feel it.

Sounds like you have leg strength issues. You need to do alot of 2 point riding and alot of posting without stirrups. You also need to start riding with feel. If you truley "feel" the ride, you should be able to "feel" the correct diagnal, therefore helping you to balance yourself to post on the correct one. Have your instructor put you on the lunge line, trotting and doing it with your eyes closed and feel the ride. It will be great for your back too because you'll be forced to keep yourself balanced. I'm a croocked rider myself so I feel for ya. LOL.

Think about it this way: like I said, posting on the left diagonal is the exact same as posting on the right diagonal. You don't move your right leg differently, your weight isn't bearing on one leg more than the other, they're the same. On the left diagonal you raise your seat when the horse's right front leg raises, and lower it when the horse's left front leg raises. Right raises, left raises, right raises, left raises. If you skip the first step--when the right raises--you have: left raises, right raises, left raises, right raises, which is POSTING ON THE RIGHT DIAGONAL. Nothing changes but the timing.

In reading all these I may have figured out something I may be doing to throw myself off  - I really do appreciate all the input.  I love getting insight from other riders.

 I agree that this isn't anything to worry about for your test, but IS something to worry about for your riding.

 I like 48's idea of using the centerline and practice changing diagonals.

I also recommend serpentines - especially during the warmup. Change your posting each time you cross the centerline. I used to have issues with feeling unbalanced the first time I would change direction in the warmup trot. And it would take me a while (2 or 3 times around the arena) before I could get my balance back. Some horses were worse than others. So I started doing all my warmup trotting on serpentines - usually 3 or 4 loops, but some loops will be shallower to avoid other horses. It might also be a good idea to throw in a complete circle to your bad direction occassionally, so it gets a teeny bit more work than the good direction.

Another good leg strenghtener is standing in the stirrups (while walking, but you can graduate to trot or even canter). Use a neck strap (or grab mane if it's long enough) if you need help staying balanced. Once you get the hang of it, there are a few "exercises" you can do. One is to raise your outside hand straight up and stretch (all the way from toes to fingertips). This is especially good for your bad side. The other (both hands down now, lol) is to shift you weight (don't lean just shift weight). Walk 2 or 3 steps w/ your weight in the left stirrup, then center (2 or 3 strides again), then right (you guessed it - for 2 or 3 strides). Switch to right, center, left. Sounds easy, but it's not!

Are you by any chance overposting to make up for any lack in momentum from the horses you are riding?  I always had issues with my left diagonal when I was riding horses and found out that I had gotten in such a habit of riding lazy school horses (and my own) that I was overposting and that was the major issue for me.  Once my trainer figured it out and got me to get my horse to move out the problem went away fairly quickly, but it comes right back if my horse isn't using himself (granted I also learned not to overpost because the first time my horse used himself and engaged his hindquarters I also over posted and got launched right up onto his shoulder so it was a pretty sharp wakeup call).  I've found that the more engaged my horse is the easier it is for me to get my diagonals and the more comfortable they feel.