Just because you know all the material doesn't mean you will ace the test, that requires both knowledge and really good test taking skills.

Losing confidence because of stupid mistakes is probably the stupidest and least helpful thing you can do here. The best solution I can give you is to practice more in a test setting (taking timed practice tests) to get more comfortable working under pressure. Do whatever you can to help you answer the questions more accurately (circle what the question is asking for and other stuff like that). Don't lose confidence in yourself though.

Let me give you a little anecdote. You mentioned general chemistry and Calc 2 right? Well I just so happen to have a general chemistry test on Monday and a Calc 2 test Tuesday. I've been stressing more over the chem test because it's only 20 MC questions with no curve. It's probably going to be full of the "which of these are correct" and nomenclature questions (neither of which I'm good with), with a curveball or two thrown in. The Calc 2 one is objectively more difficult, but having straightforward questions, partial credit on FRQs, and a massive curve makes me a lot less worried. So don't worry, you're fine, you aren't the only one who makes dumb mistakes sometimes.