The best reply you never sent is sitting safe in your drafts.

Writing something out and sending it are two separate acts, and only one of them has consequences. The relief you are after usually comes from getting the words out of your head, not from the other person reading them. Once it is typed the pressure drops, and you can see whether sending it would actually improve anything.

Write the whole thing, unedited and as harsh as you feel. Then leave it overnight without a recipient in the field. Tomorrow you can send it, rewrite it, or delete it, and most of the time you will not miss it.