Okay, so, I am a little more familiar with the sin of bitterness than I or anyone else probably ought to be. I actually consider myself to be something of an expert on the topic.
Bitterness is not a subject that gets discussed much in church-y circles. It should be, because my experiences are not all that unique or special. Bitterness is one of those ugly little sins no one wants to own-up to but that we all struggle with at some point in our lives.