If you want my opinion, stop trying to throw the disc with your arm. You are bringing the disc into the power pocket by moving your upper arm to shoulder angle. Try moving your upper arm with your hips and torso. Keep the angle of your upper arm to your shoulders locked at greater than 90 degrees. Let the lower arm stay relaxed and release like a whip after the disc comes into the power pocket. The quoted text below is from DGR and may prove helpful.
Two facts:
Fact 1: 300 ft is about as far as most men can throw using primarily the strength of their arm to propel the disc. For women it is closer to 230 ft.
Fact 2: The fact that you get the same distance no matter how you do your step implies that you aren't getting anything out of your legs, which drive your torso, which is the platform for your shoulders...
The sum:
Fact 1 + Fact 2 = You're strong-arming, throwing with your arm, and you're not getting much of anything from your torso and shoulders.
Your arm is of order 10X less powerful than your legs/torso. Stop throwing with your arm! Your arm is only useful for positioning and gripping, other than that, it is purely passive. Your arm needs to be turned into a whip that is driven by the powerful motion of your legs/hips/torso/shoulders.
Here's an exercise I might suggest:
Stand still with your arms at your side, completely relaxed. Turn your hips and torso back slowly and then rotate your hips quickly to the open position. Your arms should be whipped out and around in a windmill motion, without you using a single muscle in your arms. That's the feeling you should be aiming for.
Next do the same thing, except extend your throwing elbow out sideways from your body and hold it there (as if you put a vice around your shoulder). Allow your lower throwing arm and hand to hang limp from your elbow. Do it as if your arm were asleep and some mechanical device was locked onto your shoulder to keep the elbow pointed out side ways from your torso. Don't allow your elbow to move forward or backward, nor up nor down. It is completely locked in place, as if you no longer even had a shoulder joint and your upper arm were fused into your shoulder so that it would always point out sideways.
Now slowly turn your hips and torso back, and turn them abruptly open again. Don't use a single muscle in your arm! Now you should find that you've turned your arm into a whip. Your lower arm should be whipping forward super-fast. In fact, you can whip your lower arm forward way faster in this manner than your arm muscles could ever dream of doing. Your arm muscle strength decreases rapidly as speed increases, so they are useless anyways...trying to use them will only slow down this motion. You'll find that whipping your lower arm forward in this manner, with the elbow "stopped," will feel relatively effortless in comparison to trying to throw with your arm as you've probably been doing before.
Practice getting this feeling for a while. (Later you can work on the grip and positioning in finer detail, but for now focus on using your legs/hips/torso/shoulders as the powerful motor for whipping your arm forward
before reading these notes. I think this is a good example of where my x-step form is these days. 
