Hi Sam,
I understand it could be a bit frustrating when you see limited improvement in your performance.
Reviewing questions at the end of each exam — while it can take a lot of time — was the most beneficial part for me. Specifically, I focused on the questions I answered incorrectly, as well as those I answered correctly but left marked. For each of them, I tried to understand why I picked the wrong option, reading the detailed explanation for both the correct option and all the other options. I learned a lot from doing this! Besides that, I also tried to go back to the PMBOK and revise the topic referenced in the question.
There are many exam simulators available; my personal recommendation is to go for those providing detailed explanations for both the correct and wrong answers, as I believe this is the most useful part, especially the first times you go through a full mock exam.
I would also recommend to draft a study plan, so that you have enough time between two mock exams to revise questions and corresponding topics. Depending on how much time you have available, you may want to test on a limited set of questions.
One last thing: I noticed that my performance significantly improved by simply changing the time of the day I spent on taking a sim exam. I first used to take it in the afternoon, because it was easy for me to schedule a 4h uninterrupted session; however, after the first couple of hours, I started feeling tired and lacking concentration. When I tried to move it in the morning, it suddenly became much better! And while still being a bit tired towards the end, I could feel my mind more focused than it ever was in the afternoon. Now, this can be totally subjective, and you may work better in the afternoon or even late at night. But I strongly recommend to play a bit with different times, to go for the one you feel fresher and able to stay focused longer.
Hope this helps!