You have to first determine why you're struggling. It seems with the PS, there are two main reasons people don't do as well as they'd like: 1) a lack of deep understanding of the concepts tested, especially in the physics section that leads to under- or over-thinking questions or not being able to approach a problem at all or 2) a lack of speed in getting through the calculations, which leads to rushed test-taking and avoidable mistakes. Your issue with the PS may be one or both of these.
If it's the former, I've found that going through each concept and equation and reasoning out how exactly those equations were derived can help with learning how to integrate the given information into actually using those equations. For example, instead of just memorizing the lens equation, try to understand why those signs are the way they are and what the implications of longer focal length/image distance are. Or with kinetics, instead of memorizing several equations, just understand why d = 1/2*a*t^2 + v0*t + d0 and use that to derive any kinetic equation you need. I think this is the difference between those who approach physics or chemistry "intuitively" and those who just try to memorize the best equation to use for any given situation and end up being overwhelmed.
If it's the latter, you're just going to get faster with math, which is no easy feat. I don't think there's really a permanent solution to this other than long-term, persistent practice with doing calculations. With the limited time most people have to prepare for the MCAT, however, many people take the estimating route (where you eliminate answers based on obvious sign or magnitude issues and/or ballpark or round sums/multiplications/etc). I never felt secure using that, but that's not to say it would work for you. If timing is a huge issue, this might be the way to go since you're only going to be more nervous on the actual test. I never had less than 15-20 minutes to spare after double-checking answers in any of my practice tests but only had 5 minutes to go over my answers on test day. I'm usually not a nervous test-taker, but my head was spinning for the first half of the PS section and it took me a while to get into my test taking mode. I also had several friends who have taken the MCAT commented that they weren't able to finish the PS section on time, despite on-time performances on practice tests, so this seems to be a common enough occurrence.
If it's a combination of the two, focus on the first issue first and then tackle the timing issue. Having a solid grasp of the concepts will help you navigate more complicated calculations and determine which can be simplified. Obviously, these aren't the only two issues that come up with the PS and there are several ways of approaching the section, but this is just what I've gathered from preparing for the test myself and helping others prepare as well. However you choose to tackle the MCAT, however, it's always going to be some variation of: 1) identify fundamental problem 2) address problem 3) practice 4) practice 5) practice and 6) repeat 1-5.
Good luck with your studying!
