It’s happened to all of us, Kat. I find the easiest way to avoid publishing before submitting is to follow these steps:

click Publish but only add tags
close that screen by using the X in the upper-right-hand corner
then go to the 3 dots at the upper right corner of your story/poem/piece
click Add to Publication
choose Publication
click Add Draft
you will return to your piece
click Submit

Since I started following these steps, I haven’t accidentally published anything before submitting to a publication.

Maybe this will help . . .