"A bitter pill to swallow" is one of several idiomatic expressions that are used to describe an event or situation that is extremely disconcerting and difficult to accept. The idiom itself invokes the image of having no choice but to down some type of medication that has an extremely bad taste, with that terrible taste making the process of swallowing very unpleasant. Just about any situation in which someone has to deal with circumstances that are less than pleasant can be described as a bitter pill to swallow.
At times, English sayings of this type are used to identify situations in which an individual has no control over what is happening, and has no choice but to attempt to reconcile with whatever comes to pass. For example, if a romantic partner indicates that he or she no longer wishes to be involved and there is no chance of changing the person’s mind, then the jilted partner has no choice but to accept that the relationship is over and attempt to move on. In essence, this person has a bitter pill to swallow before it is possible to move through the situation and begin to heal from the hurt.
The use of a bitter pill to swallow is not confined to situations involving love and romance. When a company chooses to downsize or even close its doors, the employees who lose their jobs have no control over the event and must learn to accept that they no longer work for the employer, even though it may be very difficult to accept that fact. Failing to receive a job offer after being very sure that one is forthcoming can be a bitter pill to swallow for an enthusiastic job seeker. Managers or supervisors may find themselves with a bitter pill to swallow when employees they attempt to mentor fail to live up to their promise.
Even relatively mundane events in day to day life may constitute a bitter pill to swallow. A teenager who is denied use of the family car after failing to observe household rules may find the situation hard to accept, especially if the car was needed to go on a date. Having a loved one forget a birthday can be very hard to deal with. Essentially, regarding any event in which there is disappointment and difficulty in accepting what has come to pass, this particular idiomatic expression may be used to convey some idea of just how difficult the individual is finding it to accept the situation and move on.