> The issue is that my mistake stems from my carelessness, not from my knowledge of English grammar

I remember one time when I asked a native speaker to proof-read an article I wrote. Most of my mistakes were missing "s" in the present tense, one of the first thing you learn as a beginner.

> I sometimes think about sitting down and properly learning English. But for some reason, I think that would be a waste of time.

Same here. My company would pay for English lessons (3h a week with an English coach). It's quite a commitment and I'm worried that I'm way past the diminishing returns and it would take tons of effort for barely noticeable improvements. On the other hand, I'd love to get better and it'd be helpful professionally for sure.