Greenandwise. Dude, don't be so harsh on yourself. Take it from me, I just got out of the GMAT 4 hours ago. Like you I too got a 580 on my Kaplan test that I took last Sunday. Like you, I started introspecting and worrying about my chances on the GMAT. Yesterday morning or 24 hours before the GMAT, I did PP2. Got a 750 on the PP2. I know you are waiting for my GMAT score... 710. (Good enough for me, at least)
I will put a longer email thread on my experience, but not right now. I need to relax have a drink and play with the kids who I have hardly interacted with in the last few weeks.
So, just go in there and just beat down that test. Don't worry about Kaplan. Kaplan is great to build stamina and to improve your speed at answering questions. GMAT questions, at least for me, are much easier. In quant, make sure you have solved enough algebra, and integer properties questions, and that when you see the answers that you understand why you got it right or wrong. I had very few geometry questions and lots of probablility and perm/comb, more than I had hoped. Some of them must have been experimental

. Even then the answers seem more straightforward than Kaplan. In the case of verbal, the CR's were also simpler than the Kaplan set that I worked through. I especially think that Kaplan throws many answers that are just borderline out of scope, while those on the GMAT tend to be either completely out of scope or the right answer.
Anyway, good luck!!! Here's to your 700+ score.