I think that in a lot of disciplines, you have to become fluent at manipulations, and at seeing and thinking in higher level patterns. Being able to think your way through a more complex problem would seem to benefit from, if not require, seeing multiple steps ahead in a progression. At least this is my perception.
My experience in school math was that it wasn't enough to satisfy myself that I knew how to solve a problem. I then had to work my way through a whole bunch of similar problems until I could perform the manipulations quickly. This is also how I managed to commit the definitions, axioms, and theorems to memory. If I didn't do that stuff, then I got my arse handed to me on the exam. I gave my kids pretty much the same advice.
https://www.adaa.org/living-with-anxiety/children/test-anxie...
There's no shame in it (although high schoolers can be assholes), and it can help your school accommodate your needs eg extra time to take your exams.
My accuracy on the questions I got to was very high, I just couldn't go fast enough to complete enough questions. Same deal on SAT type math papers too.
Truthfully, my mention of accommodations isn't for you--I'm assuming you're out of high school already and you've found a professional niche that works for you. I mention test anxiety and the professional workarounds for high schoolers or their parents.
*) This is actually moving well beyond my expertise. I know about the existence of test anxiety, and I've accommodated students with it.
A brief consult with Dr. Google surfaced some clinically recommended accommodations that probably would have helped you. Sorry I don't have a time machine!
http://www.2enewsletter.com/article_2013_05_slow_processing....
This one's my favorite: "Reducing the number of tasks required to demonstrate competence (such as 5 math problems instead of 25)"
Source: I'm slow but good at Math and ended up dropping it as soon as I could because it would not get me the grades I needed to enter a top tier university.