That blue button at the upper left corner of the compose box that has A/A is your requested doorway. Hit that button. That's your preview option. It allows you to edit as you compose, viewing the post as it would be, for the most part. There are minor differences, but you'll get familiar with them. The Preview button in the lower right, below the compose box, will be the post as it'll be shown, but you won't be able to edit in that field. That little blue button is the magic one.
To be honest, I flub the edits all the time, so even with this help I still edit like mad after posting.
*What would I do without you...don't answer that...lol. Thank you so much!