If a student in a language class does not want to speak, the teacher will normally assume they are shy, and will encourage (sometimes force) them to speak. He’ll tell the student: “Speak and don’t worry about mistakes”. After all, speaking practice is more important than making mistakes, isn’t it?
Mistakes are not harmless. Every time you say an incorrect phrase in a foreign language, you increase the probability that you will say that phrase again. Therefore, if you speak with mistakes, you can easily teach yourself bad grammar.
A lot of people think it’s more important to improve your fluency than to improve your correctness. If you follow this path, after some time you may find you can speak the language with ease, but it will be largely your own version of the language — not correct language as it’s spoken by native speakers. In other words, you may become quite fluent in speaking the wrong way.
Perhaps such a state is more desirable than not speaking at all. But when bad grammar comes to your mind so easily, it becomes very difficult to improve. When you’ve been saying things like “He go away” for the last two years, it’s not so easy to start saying “He went away” all of a sudden.
The point is that it’s hard to go from “fluency with mistakes” to “fluency without mistakes”. It’s much easier to start with “careful, correct output” and then work on your speed to achieve “fluency without mistakes”.
Making mistakes is not OK if your goal is to speak fluently and correctly.
If you’re a person who can’t help but make a mistake in every other sentence, you definitely shouldn’t speak yet. Even if you have a teacher who will correct your every error, you will not be able to remember such a large number of corrections and will make the same mistakes over and over again. You should rather concentrate on reading and listening, no matter what your teacher says.
If you decide to speak, do it carefully, sticking to phrases that you’re absolutely sure are correct.