Do other non-native english speakers have trouble solving certain kinds of problems?

Were the advanced textbooks in English or in X? I understand that translating all the mathematical names between languages is difficult and often not at all intuitive.

"Reading definitions and theorems" is fruitless labour by itself; when a teacher brings them to life in your imagination, and you do a lot of exercises using them, only then you can understand. That's the way to comprehend "worded definitions, theorems and problems".

Try sitting into a math course at a close-by university, and make sure you also attend the exercises. You could also try books of the other language, but make sure you do the exercises.

If you already have a problem you try to solve, applying the theorems and comparing with exercises might also help (tackling the problem plus understanding theorems + problems). This applied learning might also be more motivating than dry reading.

Some things are also plain hard, so make sure you don't set your slope to steep. No harm done going back in the book to studying simpler things once in a while. All the best!