For example, while "You are a terrible communicator, and I feel like you are always talking down to me," is true, what you want to say is "You smell so bad that even your grandmother hates you!"
I once wrote a letter (never sent) to a boy that said, (among other things, because the actual letter was seven pages long) "I hope your apartment building gets hit by a meteor and (roommates' names redacted) have to eat you to survive*. Also, your hair looks like pubes.**"
*Not actually true but correctly representing the rage I was, at the time, feeling.
**Kind of true, but mean.
This is what my feelings are feeling right now:
Dear Horrible Person:
I hate you. You are literally Hitler. Also, you are fat.