What exactly is writing?
Writing is two very different things. First of all, writing is storytelling. You are telling a story to someone in the form of the written word. And it is this portion of the process that you probably derive all of your satisfaction. Secondly, writing is a communication skill. This skill is what enables you to tell your story effectively and with the tone, timbre, mood, and atmosphere that will produce in your reader the thought and emotion you wish to achieve. It is in this second half of the process where the grammar comes in to play. It is a critical component of effectively communicating.
So why shouldn't you worry about the grammar?
Because in order to go from storyteller to effective storyteller you have to write -and you have to write a lot. There is no way around it. But if the writing is an ordeal of editing and grammar checks how enjoyable will it be? And how much of it will you do? The fear becomes a road-block to what you are trying most to achieve. It causes you to write less and when you write less your improvement in grammar is lessened. It becomes a self-fulfilling prophecy.
The improvement comes just by the doing
Human beings are amazing creatures. In order to improve at something we just have to do it. To understand this concept lets compare writing to golf. How hard is it to grab a golf club and hit the ball? Not that hard right? But how hard is it to go out there and play golf well? That's a whole different story. And what do you have to do to achieve the status of good golf player? You guessed it -you have to play. And writing is exactly the same. It is very easy to write yet it takes a lot of practice to write well. And what do you have to do in order to improve? Right again! You have to write. You have to lay a lot of words down on paper. And the beautiful thing about writing is that it is exactly like golf or any other endeavor that we as human beings undertake. The more you do it the better you get. So the more you write the better a writer you become. Your inner ear improves and you start to make distinctions that feel and sound right and it's almost effortless. There are things inside you that take over and guide you. But they won't work if you don't write. Write anything at all and the improvement will come. Just write and don't worry about the grammar!
But how will you know your writing has improved?
Seeing the improvement in your writing can be a difficult thing to do. In golf you can add up your score at the end of the day and have concrete evidence of your improvement. In writing the tallying is much more nebulous and much more dependent on opinion. And this uncertaintly leaves a lot of room for doubt and insecurity to creep in. But don't let it creep in. You have to know that as long as you are doing it you are improving. It's a guaranteed thing that we as human beings can trust. And this innate ability that you possess as a human being is what will take you from the first stage of storyteller to the second stage of effective storyteller. And after hundreds of pages of and thousands of words you will have the confidence to know that you can write and then you will be able to work on perfecting your grammar. But until you get to that stage don't worry at all about the grammar just put the pencil to the paper and write!