Based on the information you've provided, it sounds like you're a strong math student overall. My guess is that you're probably in a math class right now that involves higher-level math than the SAT, so you tend to assume that problems are more complex than they really are. What this comes down to is strategic practice.
When you take practice tests, have you been keeping track of the problems where you make these mistakes? You should be able to detect patterns in the types of questions that represent your biggest pitfalls. By studying where you went wrong and then redoing these questions, you can reinforce the simpler solution method in your mind and will be less likely to make the same mistakes on future tests.
Even though these are "careless" mistakes, they're still legitimate problems that won't be fixed just by taking practice test after practice test without analysis in between. I guarantee that if you dig deeper into the types of problems where these mistakes happen, you'll have greater success in side-stepping these issues when you take the test for real.