I had one serious retardation at school, and it was maths( others would say I had many )...
I haven't run out of fuel, been lost etc. As my favorite aviation author wrote in "fate is the hunter" , " the time for happy guesswork is over" meaning all the hard calcs are done with whizz well or calculator.
You'll pick it up as you, and it's surprising how little you'll need a whizz wheel as you go on..
the mental Mathis book you've ordered is not bad, but get a highlighter, and mark the ones that are useful to you.
Have fun.