Because of this, my leg slips back, and I am waaay in front of the saddle. On the bright side, I have a big release since I'm on his neck
Jumping ahead does not cause your lower leg to fling back, nor does jumping ahead cause you to be on your horses neck - that happens because you have no solidification in your lower leg, and heels, which is what is creating the ripple effect.
Your lower leg flings back, because you are pinching with your knee's because you are not secure in your tack, which blocks the natural weight flow that occurs from your shoulders to your heels. When you block that flow aka knee gripping, blocks that flow and that's why your lower leg flings back.
As already mentioned - to fix this issue, you need to do alot of 2 point work and stirrupless work. 2 point, 2 point, 2 point and then do more 2 point and when you are doing doing 2 point work, do more.
To stop the jumping ahead thing - is to stop riding the fence, and start riding your horse.
Work on learning to feel your horses rhythm, obtaining rhythm, and keeping that steady fluid pace. Feel it, trust it, establish it.
Also, learn to trust your horse - you cannot jump that fence for him/her, that's impossible. You're the passenger at that point, it isn't your job, it is your horses job. Your job is to get your horse to the base of the fence in a controlled, safe, strait, balanced, steady rhythm and to stay out of his way. His job, is to get the both of you over that fence.
You have no reason to look at it. You know it is there, your horse knew it was there before you were even 5 strides away, so why are you staring at it? Ride your horse, not the fence.
Have someone put you on the lunge line. With no reins. Start by going over trot poles - establish a functional 2 point position, where you are balanced over your legs, heels deep absorbing your bodies weight, legs stuck at the girth, tail bone tucked under you, core activated, open chest lifting your heart, head up and looking forward and just stay there - riding your horses rhythm.
Now, work on allowing your horse to move you. Your horse is to move you out of your tack, not you. Your horse is to close the angle, not you. You just stay out of his way, by remaining solid and functional.
Then, when you get how that is supposed to feel, start going over cavaletti's. Same idea. Ride your horses rhythm, and trust your horse to do his job.
Continue to do this until you are ready to graduate over xrails and fences.
Learn to ride what is under you, not what is infront of you. Trust your horse, establish rhythm and work on allowing your horse to do his job.