Inference questions are (usually) the hardest questions on CARS, so your struggles are not unexpected. An inference question can be asked about any aspect of a passage, so formulating a one-size-fits-all approach is practically impossible. The goal is to find the answer choice that best fits the Goldilocks principle: not verbatim, not too much of a stretch, but just the right amount of an inference.
My first recommendation is to flag the inference questions and make sure you answer every non-inference question first. I know that sounds like giving up, but there is no shame in making sure you get every easy question right and using your left over time on the hard questions. On the actual MCAT (and AAMC's practice material), you won't find a correct answer choice to an inference question that is overtly stated in the passage. By definition, that isn't an inference. My other advice is to pick the answer choice most consistent with the main idea. Rarely can you infer something that disagrees with the author's argument. For the inferences where the question asks about "an assumption underlying the author's argument," it can help to approach it from the perspective of weakening the argument. If you ask yourself which answer choice, if found to be wrong, causes the most damage to the argument, this will likely be the correct answer.
Of all the recommendations, the most important one is to pick the answer choices that the author would most likely agree with. If you use the main idea/author's perspective as a guide, it can help you from being led astray.