Heyo,
I've got to say, a 570 when you're just randomly clicking through your last ten questions is pretty good. But, I understand your feelings on the score and here's some good, home-cooked advice on pacing and how to build your stamina.
1) Make sure that every time you sit down to do a GMAT question (whether it's in a problem set or on a diagnostic test), set yourself a maximum of 3 minutes to do the problem, and if it's a Sentence Correction, then 1.5 minutes. Get used to doing problems quickly and efficiently (but not carelessly). The only way to improve pace is to crack the whip and stick to it.
2) To build stamina, work as if you're under test conditions for a sustained period of time. Do two hours of practice each day, splitting time between doing problems and reviewing answers. Make sure you know two things by the end of each session: 1) what you do and don't know, and 2) why you got some questions right and others wrong. Do you find yourself getting distracted during the test? Where do you find you lose time? Is it number crunching or something emotionally-related?
3) In order to improve your speed, practice practice practice. Set a schedule for yourself and stick with it. In addition to all the other things the GMAT tests, it's a big assessor of time/energy/stress management skills. In order to do well on the test, you've got to carefully balance yourself. I can say this from experience: I left the last two questions on my quant blank and my final quant score was I think a 41, or 69th percentile. Not great but not terrible either--at the same time, I was really surprised at how time snuck up on me. And I realized when I finished that I had kind of went into a daze during a problem in the middle of the Quant section.
4) Do you spend a lot of time trying to figure out how well you're doing in the test? As in, when you get an easy question (or a question you feel is easy), do you kind of panic and wonder if you got the last question wrong? I only ask because I know I used to spend time fretting about that and it was a big time-eater. When I'm only focused on the question at hand, I do a lot better and don't spend any energy being distracted.
Anyways, hope that helps. Good luck!
Michal