You are a self-taught computer and its application (undergraduate) subject. I just finished three courses: advanced mathematics, discrete mathematics, probability theory and mathematical statistics (II). My personal feeling is:

From simple to advanced

Advanced mathematics

Higher mathematics has the most explanations and is the easiest to learn. Discrete mathematics is second and more difficult to learn; Probability theory and mathematical statistics hardly give proof, only formulas are the most difficult to understand. But if you are willing to study hard, these three are not difficult.