One of the biggest thresholds is the thought that someone else has already done that. And probably done it better, to boot. The sheer volume of other people writing the same thing as one is about to write can be offputting, to say the least.
There's no point in denying that things have already been done. The world is indeed a big and old place.
But you haven't done it. And that means more than one would think.
I tend to use three lines of reasoning to get over this particular threshold. Let's simply call them 1, 2 and 3, for the sake of simplicity.
1. Even if someone else has done it, that does not make your own take on it null and void. Even if you agree whole heartedly with what someone else has written, they still don't express your view on it. And, moreover, they don't tell those who write what you read what you think.
Your readers do not read most of what you read. And would most likely very much like to be introduced.
2. Even if someone else has done it, and done it consistently over a period of time, it still does not make your own take on it null and void. It just makes you a part of a movement (with whatever degree of formal organization), and moreover lets both of you find comfort in the fact that the other is around doing the good work.
Don't let agreeing being an unagreeable threshold.
3. Even if someone else has done it, you have something they don't: the chance to do it again. With the added benefit of their precious effort, and any eventual constructive criticism that might have followed.
Second chances to learn by other's doing is a good thing.
So. Don't be discouraged by discovering that the thing you've thought about writing suddenly gets written by someone else. Take heart and go forth anyway.
Because if you don't say what you mean - who will?