Why is learning to write well so slow, so difficult, and so frustrating?

Why is writing so difficult?
For many reasons. For example, writing is not natural, like talking or walking, so it requires a special effort. It also requires careful attention to lots of annoying details like punctuation, grammar, and spelling. Many of the ‘rules’ for English punctuation, grammar, and spelling don’t make much sense. Moreover, writing requires you to do two conflicting things at the same time: think about what you are saying, and also think about how you are saying it. And so on. To put it briefly, writing well is one of the most difficult human skills.

Why is it so frustrating?
Because usually, improvement takes a long time. Even worse, improvement is not gradual. That is, if we made a graph of writing improvement it would NOT look like this:

Instead, a graph of writing improvement would look something like this:

The second graph helps to explain an almost universal experience: you work, and work, and work, but your results don’t improve. Why? Because you are on one of those long, flat portions of the line. If you let the frustration defeat you, then you give up trying and you never reach the next level. But if you remain patient and determined and keep working, you will eventually reach that point where everything you have been working on suddenly ‘clicks’ and you jump up to the next level.

So now you want to know . . .

How does one become a good writer?
It’s not so complicated, but it’s not so easy, either.

1. Read! When you read, you learn what words and sentences look like when they are written. If you read very little, your understanding of what written words and sentences look like remains weak and uncertain. How is that word spelled? Should I put a comma here? What’s the right way to use a dash, or a semi-colon? Hundreds of such questions confront a writer, and it’s impossible to learn the answers to them by memorizing a grammar book. However, someone who reads a great deal learns to answer most of those questions correctly simply by seeing so many words spelled and so many sentences and paragraphs written correctly. So there’s the first key to success as a writer: good writers are voracious readers!
2. Pay attention. Once you become an enthusiastic reader—when reading is a pleasure, not a chore—you can begin noticing how writers write. After all, if you enjoy reading, then you enjoy the results of someone else’s writing. At first, and for a long time perhaps, you may be content just to benefit from other people’s hard work. But if you’re lucky you will develop an interest in the craft of writing—how do they do that?—and want to try it yourself. You may discover that, although writing well is very difficult, the satisfaction of succeeding at it is very great.
3. Write! You learn to walk by walking, and not giving up even though at first you fall down a lot. You learn to play the piano by playing the piano. You learn carpentry by doing carpentry. You learn to play football by playing football. You learn to write by writing. Find good writers, and copy what they do. Then find others who write very differently, and copy what they do. Practice, practice, practice. Seek out good coaches, and take their advice. 

And as Winston Churchill almost said, never, never, never, never give up.