Any tips to help me stop this?

When I am at at medium/fast canter or a slower gallop, I tend to make an arch with my body (see attached figure, and please disregard my bad drawing ability). I try to correct my legs by bringing my feet forward, but after a 7-10 strides I find myself back in this arch. Any ideas on how to fix this?

You will not be able to bring your legs forward as long as your upper body is also back behind the vertical. It's a simple matter of physics. You HAVE to keep your legs back their in order to counter act the upper body, or else you will fall backward out of the saddle (you probably don't fall because your are gripping like a demon with your knees). That is my conjecture.

You will need to sit up straigt. My guess is that you have been taught to sit back on your pockets and have your pelvis rocking for and aft with the energy going up through it and out the front of it.
You will need to sit up more vertical and think of your upper body "riding" your pelvis. If your pelvis was a horse, and you leaned back that far off the horse, you would roll off. You need to be OVER your own pelvis and riding IT ans it rides your horse. The reason I like to think of them as too seperate parts is that they must move differently.
The lower part of the body belongs to the horse and must move with him except when retarding forward movement by "stopping" the seat. The upper body must remain pretty solid and still.
In fact, the more you allow your lower body to move with the hrose, the easier it is to have a still upper body.

Any of this make sense?

I honestly think the easiest way to fix your problem is to fix the toes down issue illustrated in that drawing. If your toes truly are down like that you're moving your upper body back to counter act the fact that having toes down takes your entire upper body forward and forming an arc with your body.
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I see what you guys are saying.

When I bring my legs forward I try to put my heel down to straighten my upper body. I feel when I am doing it, but I guess my problem is preventing it from happening.

Tinylily, do you mean I need to rotate my pelvis to a vertical postion or my upper body?

Both. When I look at your avatar photo, the pelvis looks quite vertical and the upper body is over the pelvis quite nicely. I think that perhaps, as you get going faster, you grip up with your calves, which causes you to have toes down and heels UP and you lean back too much.

So, look at your avatar and think that position , even at a good quick canter/gallop. Relax and let your legs hang down. Do I know this feeling? You bet I do! I know how it is, finding yourself gripping on. Do you lose your stirrup at canter? I used all the time, especially the right one (the stronger leg, which was doing the bulk of the gripping up)
Start riding at a slow canter , find a spot where the horse has settled into his gait and it's pretty rythmic, and RIDE him and relax and really think of your legs goind down his sides like the two halves of a clothespin (maybe an old image)

Anyhoo, just keep that image in your avatar in your brain and BE that lovely rider, even at a gallop.
Easier said than done, I know, but one works toward those things. Savor each moment of success and each one will become close to the next one, until they are nearly nonstop.

Thats EXACTLY what I am doing!!! I will usually lose the outside stirrup when I am riding (Although it happened much more frequently when I first started riding). I always thought it happened because I was leaning to the inside of the circle.
The problem isn't as bad at a slower canter, but it still isn't picture perfect.

Thanks for your help!!!