Feynmann as a teenager (and his friends too) got up close to the state of the art in their day (Dirac's Principles of Quantum Mechanics - the bible for a generation and a half) by studying the "...For the Practical Man" series (Algebra for the..., Geometry for the..., and he had to wait for it, but eventually Calculus for the...). Last I looked these were still in print, but renamed PC style "for the Practical Worker".
There are a lot of books that take you through high school and lower division undergraduate math. Browse the math and science section of any big book store. Pick the ones that appeal to you best. And follow these rules (or at least feel guilty about not following them)
1) Don't skip. In math everything depends on everything else, and by definition, you don't know how it all hangs together.
2) Do the problems. Or at least the problems you can do. If you get stuck, give thanks you live in the computer age and post it here. Someone will help you.
Set yourself a schedule with each book, not too daring but enough to keep you at it. One section a week? Plug away. Periodically go back to sections you did weeks before and see if you can still do the problems without peeking. This is really stretching your mind, and not everybody can do it. When you're away from your book (like in study hall with nothing to study) run through the latest stuff you've learned in your mind and try to turn it backwards to see if you really understand it.
Best of luck and keep us posted on your progress!