Descriptivism and Prescriptivism
In the field of linguistics, there are the concepts of descriptivism and prescriptivism. Descriptivism is about describing how language is used. In contrast, prescriptivism is about prescribing how language should be used.
Linguistics, as a science, must necessarily be descriptivist. You don't learn about something by telling it how you think it should work. To learn about the world, you need to observe the world without judgement.
But that doesn't mean there's no place for prescriptivism. In fact, I would argue that prescriptivism is unavoidable, to a degree. When you speak or write, you have to choose what you're going to say. Many of those decisions happen unconsciously, without deliberate intent, but not all of them. Descriptivism alone can't tell you what words to use.
Where prescriptivism goes wrong is in saying that you should speak a certain way because it is the right way. The problem is that there is no single right way of speaking.
Consider, are people who speak a different language wrong? That seems patently absurd to me, and I think most people would agree. But if using different vocabulary and different grammer isn't wrong when you're speaking a different language, why should it be wrong if you're speaking a different dialect?
Furthermore, there's no clear line between a separate language, and a dialect. Typically, two different ways of speaking would be called separate languages if they're not mutually intelligible and dialects if they are. But consider a case where group A can communicate with group B, group B can communicate with group C, but group A can't communicate with group C. This is called a dialect continuum, and is actually fairly common. Do groups A and C speak different languages, or dialects?
And how do you determine which way of speaking is the correct one, anyway? Historically, it was just whichever dialect the monarch or nobility spoke, but why should their opinion matter more than anyone else's today?
Instead of trying to speak correctly, you should try to speak in a way that maximizes your audience's understanding. (At least if you're trying to communicate, which is usually the came when you speak.)