It sounds like you just might have some sort of diagnosable medical condition that is creating problems for you when you take the SAT.
I had long suspected that I might have such a thing, so when I went to law school I went to an expert and got completely evaluated. He did find a diagnosable condition, so he sent the results of his evaluation to my law school, together with his recommendations, and the law school therefore gave me 50% more time on all my exams as a result. So when other students took an exam that took 2 hours and 45 minutes, I was given 4 hours and 7 minutes.
In America we have a rather strict law called the Americans with Disabilities Act that requires many places, including educational institutions, to provide "accomodation" for people with disabilities. Like the extra time on exams that my law school gave me.
I would suggest that you get a similar evaluation, and then find out what sorts of accomodations the SAT has for these sorts of conditions.
(Interestingly enough, I am kind of the opposite of you. I had no time trouble with the American SAT, but I did with British O-levels.)