In human relationships we start out on the teeter-totter of a fulcrum.  When one side of the relationship does something painful that affects the person at the opposite end of this balancing act the tetter-totter can become a slide.  At which point you must choose either to jump off or work harder to keep your balance so as to not go down the slide into anger and bitterness.  You may need to remove a harmful person from your life, jumping off the slide, as it were.  However, in life few things, if any, are worth punishing yourself with bitterness.  Yes, I said punishing yourself, because when a person spirals into anger and bitterness it is self-harm.