Big O and time complexity are not very difficult mathematically. Simple arithmetic. It is just a way to judge effiency of an algorithm without factoring in outside concerns, like hardware, and operating systems.
Logic and induction, which is a large part of the foundation of mathematics, is not all that difficult either. If you can understand basic logic principles,
you should be ok. Recursion is very closely linked to induction. To get a small taste of induction, google for inductive proofs of simple series and sequences like factorials, fibonacci or towers of hanoi. They are simple, but likely are things you will see at this level of coursework. It is just basic logic and recursion. Google Big O notation as well. It should ease your mind a tad.
Other then analzying the data structures, there is not a whole lot of math to worry about.
Well, I understand programmable things like linked lists and trees. What I don't get is the math behind it.
That is like saying you understand math, but not proofs, which is very contradictory. Understanding the mathematics of computer science(Calculus, proofs, linear algebra, ect) can only help you. Have you ever written a data structure, or have you always relied on API's to do the work for you?
Data Structures is a very important topic, and I think anyone who calls themselves a programmer should know how to write efficient implementations of them. Even if you never need to write them in the real world, it will make you a better programmer. Same thing goes for higher level, more mathematically intensive courses like finite state automata, compilers and programming languages. Certificates are fine and all, but they don't give you the breadth and scope that you can get in a formal setting.
I know I ranted a bit, and I apologize, it is disheartening to see so many people in the industry who lack the theoretical knowlege of computers and the mathematics, they dip their toes in and get a certificate or two and call it good, so I tend to ramble on a bit more then necessary. It is nice to see someone diving in.