I have come to believe that I was hasty about something, and I would like to alter my review from negative to (at least) neutral, and especially to change what I said. This does not seem possible to do, which is unfortunate. Or am I wrong about that?
A quick Google check brings you to this:
Viewing and changing feedback left for sellers
Essentially, you can’t change the feedback yourself, but you can leave a follow-up comment. Maybe leave another comment saying that you were too hasty and that you were actually satisfied. Someone reading your comment should see the follow-up.
Alternatively, you can contact the seller, and tell them you want to revise your feedback. It will be up to the seller to enable that. If they agree, they will send you an email with a link, and you can click that link and change the feedback. If you left a negative comment that you want to change to a positive one, I suspect that they’d be open to such a thing.