Saying something negative about someone for the sole purpose of not keeping everything pent up may be muttar if all of the conditions of lashon hora l’toeles are present.
Of course there’s room to exist and be human, but part of being human is controlling yourself and not “expressing yourself freely” when the purpose is to put someone else down.
Again, though, if the purpose is really for emotional health, speak to a rav about the circumstances and conditions undir which it is permitted (and proper) to say what you need to say.