I have no idea how to prepare for math olympiads

Hey guys. I've been studying Mathematics and Statistics at university for two years now. I'm good at math, but I'm a complete failure when it comes to solving Olympiad problems. I decided to spend the summer practicing, but I ran into a ton of problems.

1. I found some books, particularly Arthur Engel's "Problem-Solving
   Strategies" and it turned out to be too difficult for me. English isn't my native language, and I simply can't understand the material. I just feel too stupid for it.

2. There's too much of everything. I open archives of problems from previous years and can't solve them. They're all on different topics, require different methods, none of which I know.

3. I really don't understand. I haven't experienced this feeling in years. I'm used to reading a theorem or problem and understanding it right away, but now I have to put in a lot of effort, but I still don't understand anything. I feel stupid and incapable of understanding everything needed for the Olympiads.