Bitterness comes from a continued holding onto the entitlements or prestige we expected we were due, but were thwarted by the actions of others or life circumstances.
Unfortunately, bitterness is a very ingrained emotion, built up over a long period of time. In this frame of mind, we close ourselves off from a lot of other opportunities, just so we can continue to have justification for what we feel. Typically, other people avoid triggering the emotions by avoiding bringing up the topic with us, as it can be very uncomfortable listening to our bitterness and resentment.
So, how do we break the spell? That will be difficult unless we can disconnect from the circumstances that led to the original decision to feel aggrieved. Bitterness and resentment are there because of the continued decision that the original decision was valid, resulting in a lot more emotional investment. If we can allow ourselves to talk about the trigger point of the decision, but not so that we re-enter the thinking pattern, there may be a chance to dissipate some of the emotion, so that we may get to objectively see that that decision was invalid.
If our friends or loved ones are not equipped to help us do that, a clinical psychologist might, but then we would have to be amenable to that. Otherwise, it would possibly take a heavy emotional shock of a more serious nature that would show us how much of life we have bypassed, but only life could arrange that. It is better that we instigate changes than have them forced upon us.
We can get so caught up in the emotions that we carry as a result of our experiences that we may not see that what we wanted or expected was not what we really needed, but an addiction to our desires. Step 1 of breaking that is to stop reinforcing the beliefs, and step 2 is being willing to see the truth, both of the original circumstances, and what the ongoing beliefs have done to us. Step 3 is walking away from it all!