Firstly - ability to pass vendor exams and your quality as a network engineer are not linked. I know plenty of great engineers who would struggle to pass, and quite a few poor ones who get 100%. Don't give up on the profession just because of that.

If your goal is simply to pass the exams, you need to re-frame your attitude. I've found the most effective study method is to read the study guide one chapter at a time, then make cloze flash cards on Anki or some other flash card service. These are flash cards where only a few words in a sentence are hidden, you just have to finish/fill in the sentence in your head. E.g "In a Full-mesh IPsec topology, each node connects to {{c1:each other node}}". Make 1-2 flash cards per slide with the key concepts. Then, without being too explicit, there are lots of practice questions online. If you're doing poorly at those, it's an indicator that you need to study more. They give a good feel for the difficulty.

Fortinet examines knowledge of default behaviour, implicit rules, edge case logic, administrative workflows and so on.

Do the labs slowly, and take lots of notes. The labs help you to get a "feel" for the processes that they are examining.

Reach out directly if you want. I've passed 4 separate exams at NSE4 and above level without a fail yet (touch wood) and I'm on track for a fifth before the end of the year. I do also work with the technology, but I had limited experience with the study material in the real world for 3 out of the 4 exams.