There are things in my life I feel bitter about (my childhood isn't one of them). They are things I feel I can't do much about, however much they affect my life, and which I didn't cause, and that is the source of the bitterness. I know there are better ways of dealing with them than bitterness, and sometimes I even manage to do that.

It sounds like dealing with the issues is causing your bitterness. It's an unpleasant feeling, but it should be possible to come to terms with it at least partially. It's not wrong or unnatural to be bitter - it just feels dreadful.