Your English is not where you want it to be, and it has nothing to do with you not knowing the top 5,000 idioms.
When you’re under pressure, tired, or just hungry, you won’t be at your best. That goes for your mother tongue as well, but the problems are clearer and more pronounced when speaking English.
That’s because your own language is automatic. You have a feel for what is right. There is no logical thought process or analysis that slows you down.
For example, do you ever freeze in English? Forget what it was you were trying to say? Make a mistake with some sort of grammar that theoretically you studied decades ago?
Language in general is a natural, human phenomenon. There are over 300 million people in the US – we’re not all geniuses, but we’re all pretty damn good at the 2nd conditional. That’s because the 2nd conditional is useful in daily life. If it’s not useful for you, then you have work to do.
…and that work requires automation.
You learned that 2×2 = 4, 4×4 = 16, etc. Now, you respond with what you know is right and without any logical thought process. The same should be true for English.
When you’re under pressure, tired, or just hungry, you won’t be at your best. When your English is reflexive, that will not matter.
What does that look like?
Success. It looks like success.
This is you when you’re thinking too much:
You understand the situation. Then, you use facts and logic to construct a grammatically correct sentence in your head. That is too slow. It is called the TRIANGLE OF DEATH because you will never speak fluently if that’s what you rely on when you speak.
This is you when your English is automated:
Basically, we have a situation where you’re not analyzing the situation, but you’re instinctive about what you say based on the situation.
The way to get there is just like what you did for math: go through the forms and formulas so that your brain doesn’t even consider (incorrect!) alternatives (incorrect!). When you are there, the right answer becomes “whatever, duh,” and the wrong answer just FEELS wrong.