I've done this before and it's not always cowardly. Like if someone has hurt you, writing an unsent letter can be the Christian way to deal with it. Forgiveness can be something unseen and unheard which will not only give you peace of mind, but give them the chance to change on their own without the guilt and discouragement of you pointing it out. Because it's unseen and unheard, writing it down somewhere in a letter or a journal makes you feel like you're done with it.