starting to write and then cancel my post.

I get what you mean, I do a lot of that myself. Although it's unfortunate that I often find it easier to hit send when replying to internet strangers than I do when messaging my friends. I suspect it's because online feels far lower stakes, even though my friends would be far more charitable to a poorly articulated idea than the internet would.

If it helps, I don't think you should feel bad about cancelling unwritten messages. Maybe sometimes you don't actually know enough to have an opinion on a topic, so refraining is the wise thing. Maybe other times, you have Thoughts, but they're still sort of fermenting in your head and they're not quite ready yet. Or maybe you've distilled your Thoughts down so that you know what message you want to convey, but you don't think that this particular conversation is the right time or place for them (possibly due to realising you're in conversation with someone who isn't arguing in good faith and continuing would be unproductive). These (and more) are all valid and good reasons to not actually submit a post or comment you start writing.

The advice that I try to give myself is that we're under enough pressure as it is without helping more on unnecessarily. Sometimes that pressure is because we have something that we desperately want to say, but it's hard to articulate it in a way that doesn't feel like we're dishonouring the meaning of what we intend. That pressure is hard to counter because it's coming from the weight of the thing we want to say, but I ease it by reasoning that the important ideas will find their own way out of our heads and into the world, if given time, and that they will still be important.

I figure that there's an infinite array of conversations on the internet that could've happened but didn't. It'd be a shame if we let the conversations that never ended up happening distract us from other conversations that we're actually having. Which is all to say that it's okay if you start replying to this comment and cancel it. Maybe in the next ~~life~~ thread, eh?