Your pull line is coming over the top and rounding instead of coming straight through the core. You bury your head into your shoulder which kinda throws your balance behind your heels instead of transferring your weight forward. Your weight is too far back over the rear foot/knee going into the x-step and your x-step is very deep so your weight is moving way back and never really gets forward. Stay taller and lighter on the feet but keep everything smooth and controlled and then accelerate through the hit. You can keep the disc more stationary going into the backswing and work your body more around the disc into the backswing, instead of moving the disc into the backswing. It's easier to build core torque working around a stationary object than it is moving an object. When you go down to plant you want to land on the toes and roll to the heel keeping pressure through the inside arch/instep/leg down to the heel. On the rear foot you should be feeling leverage through the inside foot/leg. Watch the reachback position and how your rear knee is behind the rear foot, you've lost leverage.