In Shakespeare's play Hamlet, what does Ophelia mean when she says that "We know what we are, but know not what we may be"?

When Ophelia utters this line, she is in the midst of a mental breakdown.  She has experienced so much heartbreak and such unpredictable tragedies that she is no longer capable of functioning successfully.  First, she seeks to obey her father, Polonius, by breaking off her relationship with Hamlet, the man she very much loves.  Then, Hamlet begins to act mad and treats her in such a degrading and often confusing and contradictory way that both hurts and humiliates her.  Worse yet, Hamlet then murders her father, and even hides the body for some time as a part of his supposed madness. 

Thus, she has lost her father as well as her lover (who she only throws over in obedience to her father).  She is bereft and alone.  At this point, having experienced some kind of mental break as a result of all this tragedy, she says, "we know what we are, but know not what we may be," indicating that we can only know ourselves as we are now, in the present moment, and we can have absolutely no idea who we will be if and when tragedy strikes.  We can never know what will come or how we will be or feel or do.