Note that I don't have these ("bitterness and unresolved anger"), but I read something this morning that reminded me that far too many do. now it isn't that I fail to struggle this area...from time to time we all do with these kinds of feelings...but I have been truly blessed in life with a capacity to learn, and learning is, I think, the key to not falling into the bitter and angry trap.
The learning stuff is part nature and part nurture.
The nature part of this equation is a bit more nebulous than it's counterpart in nurture. Two concrete examples do come to mind though when I think about myself.
1) From a very early age I've always been consuming information. I would go to bed reading the encyclopedia. I've read the newspaper just about every day of my life since I was old enough to read. I probably spend an hour or more a day reading the news on-line. I am an information junkie.
2) I've learned to change my mind about things when presented with evidence that runs counter to an established view. Personally I think far too many people confuse "principle" with "stubborn" (especially in Washington DC).
Having a built-in capacity to learn seems to be an essential tool to help you overcome those periods when you have all these feelings that you just can't seem to handle. If anything, it provides a kind of safety net of sorts: you know that if you are feeling a certain way, others have probably felt it too, so you seek out learning in that particular area to help yourself.
The nurture part of this equation is easy: I've simply been around people who were bitter and angry for most of their lives. And some of them have died bitter and angry. It seems to me that bitter and angry folks are probably pretty damn lonely, in part because they are so bitter and angry. It's a horrid, vicious cycle that I suspect gets just about impossible to break as you get older. Regardless, I decided a long time ago that I wasn't going to live my life this way. I recognize my flaws, but failing to have a positive outlook isn't one of them.
In the end, maybe being bitter and angry really is a choice. Maybe it's a choice that some folks simply don't realize really is a choice. To paraphrase that philosopher of Western culture Geddy Lee, "If you choose not to decide you still have made a choice." I think he's right (as well as being a heck of a good bass player).