Lucy, your fears and feelings in this matter are entirely understandable. But, more importantly, they are a sign of growth.
When people start out, i any endeavour, they will most often imagine it is so easy. They ahve seen how others perform, what others create. It seems so effortless. And, thus, they believe their own efforts to be on par with others, worthy of the same recognition and success.
It is only with maturity of skill - awareness of how much there still is to learn, how much effort the masters of the artform put into their creations - that one becomes aware that those initial creations may not be as good as we once thought them to be. Your fear is a sign that you are learning, that you have an awareness of the strengths and weaknesses of your own work, even if you can't articulate them.
All I can say is push on. Keep learning. Master your craft. You will find, with time, that you pick up skills and techniques that never occured to you, and eventually, they will become a natural, unconscious part of your process. And your writing will be better for it.
Of course, as Lynn mentioned, putting yourself out there, inviting the rejections, is a way to get feedback on how you are improving. Not all at once - you don't want to burn out your list - but when you feel you have something that is as good as you can make it, and have had feedback on it from other writers (and/or a JW assessment), then you should take the plunge.
Then, while you're waiting, get stuck into the next thing you want to write.