I am going to be very "Frank" With you. I had the same thing. ADHD.
ADHD one is obviously Attention-Deficit / Hyperactivity Disorder.
I have had it my whole life and I will tell you right now, If you do have it, It will be a constent challenge.
Just as you explained it, If I was told to do something, and I told them later, It would escape my mind completely. That's why I developed a habit to do it eminently or stay focused. With ADHD it is very easy for me to get side tracked on many other things.
And what you explained with school, your eagerness for Math and Science but no other subjects, and the fact that you cant pay attention, I went through the same thing. Its something you cant just put Little effort into, you need to consistently address the problem, need to slap yourself every once and while to get your mind back on tract. What I recommend for reading is finding a book. Not just any book tho, something that has your interests and are challenging, and if you can, get a nook or kindle. Without a dough my kindle has brough my reading of maybe 1 book every three months (Not school required) Too about at least Ten a year.
One more thing I should warn you, There is medicine for ADHD and when I was in 1-5th I took medicen to controll it. But there was a side effect for what I took, It zapped my energy and made me depressed all the time, in the end I slowly went off my meds, being able to controll it better. And just remember, it will be something you need to work at the rest of your life. It will get easier over time as you control your self, and it is controllable.
Any further questions please by all means, ask anything.