Sounds like the problem is that you're going for too much too soon. If you give yourself a few months you will be much better prepared for it. Patience and hard work pay off.
That said, there are a few things you want to do and keep in mind. Since we don't have a video to see, it's hard to say what specifically you need to work on, so I'll give a couple general tips and principles for long kongs.
1 First of all, you should absolutely be using a split foot take-off. No punching.
2 Speaking of the approach, you'll need speed and commitment. Long, smooth strides in your run-up will help tremendously. If you are stutter-stepping, fix that first before attempting the vault. Fix the stutter step on a similar vault you already have mastered, such as an easier kong. Make sure you're using efficient movement of the arms that helps you gain momentum, rather than robbing you of it.
3 This may seem a bit obvious, but dive as far as you can and get your hands as far back on the object as possible. The farther your hands are, the easier it is to clear. If you're not clearing it yet, try working on just getting your hands further and further each time while still landing with one or two feet touching down, then smoothly kong down off the table. As you do that, increasing speed and hand distance each time, you may find that once you unexpectedly clear it.
4 Get your hips high. They should be slightly over your head as you hit the apex of the leap, but not way above your head.
5 Practice, practice, practice --- but not until you break yourself. If you've tried it 10-20 times in one session and you're still not getting it, perhaps today isn't the day. You can always go work on something else for awhile and come back to it. I find when I do that, my head is cleared and it's easier. Many things though, I walk away from and don't try again for a few days, months, and sometimes if it's in another city or state I may not try again for years. When I'm supposed to pass that obstacle, the opportunity will arise.
Good luck. I hope my tips helped.