Bitterness to me is misplaced anger. We all have things we could be bitter about, but it's how we put that feeling to use that is the important thing.
For instance my ex abandoning me. I could've very easily become bitter and twisted, but I didn't. While I admit I went through all of the emotions associated with grief, and also much anger....I didn't let the bitterness overtake me. Some will say to embrace the feeling, others will tell you to ignore it. I would say let it manifest in a positive way. For example, you feel bitter and cheated about how you grew up....so turn that into a positive by taking yourself out, maybe with friends, have a good time...and look to your bright future. Try not to dwell on the past.