Bitterness is onerous to ameliorate. I would suggest sublimating it into something positive, such as exercise, creative self-expression, a career, caring for stray dogs, or some other such thing. The cliche that a life lived well is the best revenge is probably right.
That said, I've mostly failed to follow my own advice. I've attempted to beguile bitterness with compassion meditation. I met with some success and catharsis -- a purging of bitterness, followed by an overwhelming feeling of joy. There's something transcendental and beautiful about being able to look an enemy in the eyes and maintain equanimity and compassion. Such strength! Such forbearance! To look to someone with love in your heart as they sneer in your face provides liberation, provided such practice recurs.
As I have failed to achieve that ideal, I would not think to blame others for not achieving it. I sympathize with your difficulty in overcoming bitterness.
I frequently struggle with bitterness, as well as the resulting fantasies of vengeance. I hope to one day harness the power of compassion to free me from bitterness and animosity.