How hard it still is to read English text! On the other hand, I can read Japanese text with ease. I feel the processing of the text is vastly automatized in me. That is, meanings of sentences are presented to my mind with little or no mental effort exerted. This must have been realized by quantity. The quantity I processed as a native Japanese speaker. This contrast in the reading speeds shows that my reading English is still not enough in quantity.

Humans are generally professionals of a language. They have been using one (or several depending on the person) language every day since when they were babies. If one has a career of mathematics of the same level of experience, they must no doubt be called at least a professional, if not a genius. Mathematics at a professional level is hard to acquire. So this could be a story that indicates that a foreign language at the native-level fluency and understanding is hard to acquire.

In learning English, when you feel a bit tired of reading English, it might be a good method then to switch to writing English. Output after input. It's reasonable. Not vice versa. Perhaps, within you, something requires you to do the output when that thing judges it's the time.

Repeat the cycle: input, output, input, output, ...

When you don't have a conversational partner, you can write as an output practice. It would be helpful to use Google Translate. You write English into the left board and the translation instantly appears at the right board. And by reading the translation, you can check whether what you meant was "understood" by the partner. Here I used quotation marks to the word "understand," the reason of which is that, of course, machine does not really understand it. Plus, it's because machine translation does not always give a sound translation even if given a grammatically and contextually correct text.