My muscle memory is having a stupidly hard time shifting from Reddit to Lemmy, so I keep typing out comments I've put careful thought into, and at the last minute hitting the Cancel button!
No it's because Cancel is where Post was (or Send or Reply or whatever it was called, I forget and I'm not logging into Reddit just to check.) I never had much problem with trolls, being anonymous helps it roll off one's back.
Surviving the server overload and memory leaks around and following July 1st taught me to copy any long post to clipboard before pressing post, and this habit has served me well.
Otherwise if you haven't checked it out already, the Voyager app has a confirmation prompt before cancelling post.
I got so many error submitting comments early on, so whenever I write a lengthy comment, I always copy it first before hitting the post button, just in case. Saved me from typing again a couple times.
The app I use (Voyager) has a "are you sure?" sort of feature where a window asking if I wanna delete the comment or cancel the cancel, as in undo the cancel and continue writing the comment, pops up. Maybe you could give it a try and see if it helps prevent this issue lol.
Are you using an app or a website? Apps might help here as there are rules and defaults for each platform that could put the buttons where you expect them to be.