Study.
I know that sounds flip, but that's what you have to do. Do not concentrate on how many times you have taken the test. Convince yourself that this is the first time.
Study. Study harder than you ever have. Do practice drills. Time yourself, see how quickly you can answer easy, medium and difficult questions. Read about science all the time -- not just in your class books, but in the newspaper or online. There's always some sort of news story about NASA or dinosaurs or geology. Read, read, read and make it a part of your life. One thing about studying is, if you hate what you're studying, you may not retain as much because you won't want to bother. You may find yourself putting in the time but not getting much out of it. So try to make your peace with science because then at least if it can become somewhat pleasant to read about, that may help you turn the corner.
Math is harder to embrace so you'll need to do some fancy things. Do you like sports? Then calculate winning percentages for your favorite teams, or figure out who's going to be the first team to be mathematically eliminated from the playoffs. Bake cookies and double the recipe. Next time, halve the recipe. Look at your bank balance and figure out the interest rate. Look at numbers outside, anywhere, even the route numbers on the highway, and try to figure out things like if the number is prime, or if it's divisible by three or what would happen if you multiplied it by six or what's the square root.
Study.
You can do it.
Study.