Since you got a 650 years ago, obviously at one point you had a good understanding of the concepts. It sounds to me like you just need a refresher course on some of the basics, and it should come right back. The math especially... it's like riding a bicycle. Seriously. I hadn't seen any of these concepts in years (since high school) but once I started crash-coursing, they came back to me within a week or two.
Just pick up some study materials and work your way through them. You have what it takes... you just need to remind yourself.
Good luck!