Like was mentioned, Khan Academy is a great resource. That said, I'm finishing Mathematical Reasoning on Study.com. As I have largely neglected math since middle / high school, I've taken time to try to relearn the concepts presented. The trick to learning it and developing competency, I think, is (like davewill said) is repetition. I've also taken tons of notes (thirty six and a half pages) to reinforce the information. Good luck!
MSK9, MD
Resident Physician
Resident Physician