To be bitter means, disappointed, disillusioned, and pissed about it. Disappointed because you thought something or some one could make your happiness for you. Disillusioned because that isn't true And pissed because you don't realize the truth about it. oh and too much sour not enough sweet.
To be bitter is to let it get to you. To let it fester--whatever it is. The inability to forgive, to let go. Double edged blade.