Oh my goodness. I feel you!
I get horrible test anxiety. For the SAT, one section I was shaking so bad that I couldn't write. During another section I fell asleep (I have narcolepsy). Tests stress me out like none other. Even the anticipation for it feels like I'm getting an ulcer.
It sounds like it's more like, you read the question wrong, or your stressed out about time, or you worry so much about whether or not you chose the right one that you have to change your answer. Thats what it is for me most of the time. Is it test anxiety that your talking about?
If it is, I've found that these help a little:
Amazon.com: Rainbow Light Busy Brain Release, Coenzymate B, 60 Mini-Tabs: Health & Personal Care

Also, sometimes, simply just taking the time to count your breathing. In four seconds, Out four seconds, and do that four times. It may seem like you're wasting precious time, but it is much more important that you're able to focus. So no, you're not wasting time.
And, if you have time, read each question twice, to make sure you're reading it right. Because if you can't figure out the answer you'll probably read all the answers over and over again, but did you make sure you understood the question first?

I don't know. May not make much of a difference, but at least it's a start.

Good luck!!