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(haha, I win this time)

The idea of the "L shape" is not in and of itself incorrect, but what it is is an aesthetic concept that doesn't give you an explanation of why, so it's hard for you to use it effectively, since you don't know what it's supposed to be affecting. That might have been confusing...

Your arms, while riding, should be in some sort of L-type formation, with your elbows bent and soft and your arms forming a straight line from the bit, through the reins, to your hand to your elbow. This will make kind of a... slightly open L. Your elbows should be just slightly in front of your torso, think about level with your belly. (well, my belly. From your pictures, you're skinnier than me razz so just pretend you have a belly) Your reins should be short enough that you can feel contact with your horse's mouth, think about 5lbs of resistance. This allows you to use smaller motions in your hands to communicate with your horse, rather than having to use larger gestures like puling your entire arm back to turn him. He can feel the twitch of your ring finger if you have contact and he is soft on the bit. Keeping a straight line from bit to elbow is important in this communication, because if the line is broken, the contact is broken. This broken contact may give you more leverage, but it ruins your communication.

A lot of people will tell you to put your hands "forward and down" if your arms seem to be in too rigid of a "L" shape. Your elbows and shoulders are tense and your thinking more about the appearance than the functionality of the position. When I'm teaching, there is a difference between "Push your hands forward" and "Give your hands". "Push your hands forward" means exactly that. Loops in the reins, push your hands up onto the neck, possibly in two-point, and get our of your horse's face for a second. "Give your hands" means something much subtler and softer. Yes, you're pushing your hands forward, but only about a quarter of an inch. Think of it almost as just softening the contact for a stride or two. If you have elasticity in the contact, then the horse should follow the bit as you give your hand, then come back to you when you take the rein back up. This is what having the horse "on the bit" is about. It's about the horse trusting and listening to your aids, and following where you put them. It has nothing to do with a headset (tangent, sorry).
The problem you get with "put your hands forward and down" is you end up with open and rigid elbows. I know a lot of riders who have terrible elbows. Mine aren't always great, but they tend to go in the reins-too-long direction. If your elbows are open and locked, then you have no elasticity, which means no communication, and no control. Keep your hands up, a few inches above the withers, so you have that straight line from the bit, but keep your elbows SOFT at all times.

Something I used to use for one of my very young riders (who also had autism, so very literal in taking directions) is that you need to be sitting up tall, but still be able to wiggle. She had the problem of sitting really tensely and rigidly, but when I'd tell her to relax, she'd get super loose and flop over the saddle. As soon as I said to sit up straight, she'd go rigid in her back again (like I said, VERY literal). So think about keeping all of those position points that your instructors drill into you (they're important!) but make sure that every part of your body is still able to wiggle!
(hey, just because it's something I used with a 9 year old doesn't mean it's not relevant to those of us who are older and/or have more experience!)

Also, think about when you're posting. Do your hands go up and down with you when you're posting? No, your hands stay steady, and your elbows open and close as your body moves so that the contact stays steady on the horse's mouth. Your elbows have to be relaxed for this. They should be relaxed like that all of the time.

You also need to be careful with the "L" shape that you don't let your reins get too long and your elbows get behind your torso. This also diminishes your control, as you have no where for your arms to go should you need to use your reins. It also encourages leaning on the reins for balance, which you should NEVER do. The reins are a device for communicating with your horse, NOT a balance point for the rider! It is not fair to a horse at all to expect them to hold you up with their jaw. Some horses will lean against the bit to try to get you into that ineffective position. I usually drop them on their face at that point razz But like I said before, I'm kind of mean.

So keep your hands in that loose "L" shape, keep your elbows in front of your body and your hands in a straight (but soft) line with the bit. Keep your shoulders relaxed and down, your hands close together and your elbows close in to your sides (because chicken wings just looks funny). Keep your arms relaxed enough that you can follow any movement of the horse's head, but strong enough that you can also affect their way of going.

There's your novel for the day smile If any of that didn't make sense, just ask me to clarify.

Ugh! You guys beat me to it!

Couldn't have explained it better anyways xD