Why is Portia upset?

Although Portia is a young woman, she has already grown “aweary of this great world” (act 1, scene 2). In her conversation with Nerissa, her maid, she admits the disadvantages of having a “hot temper,” and that her situation is better than most people’s, as she stands to inherit a fortune. We assume that her father had died a while ago because she is no longer in mourning. Portia is bound to the terms of his will, and the concept she is stuck on is “choose.” She keeps hoping for sympathy from Nerissa regarding the apparently airtight terms of her father’s will: “Is it not hard, / Nerissa, that I cannot choose one nor refuse none?” Nerissa defends the dead man’s “virtue” and “good inspirations.”
Portia has already embarked on the quest for her future husband according to the will’s terms: it is a lottery system, by which the man who chooses correctly among three caskets will be the one she marries. Her father was totally confident in the system he devised, which includes hidden meanings assigned to each casket. As Nerissa puts it, these messages ensure that Portia will “never be chosen by any / rightly but one who shall rightly love.” From the suitors she has met so far, Portia remains unconvinced. One only talks about his horse, another does nothing but frown, another is frivolous, and so forth. As the German lord drinks nonstop, Portia laments: “I will do any / thing, Nerissa, ere I'll be married to a sponge.” In sum, she is worried that no one will prove suitable:

If I live to be as old as Sibylla, I will die as
chaste as Diana, unless I be obtained by the manner
of my father's will.
http://www.opensourceshakespeare.org/views/plays/play_view.php?WorkID=merchantvenice&Act=1&Scene=2&Scope=scene

When the audience is first introduced to Portia in act 1, scene 2, she is in a melancholy mood and laments to Nerissa about her difficult situation. Portia expresses her displeasure regarding her inability to choose the person she wants to marry. Portia has agreed to honor her deceased father's wishes that she will marry the suitor who chooses the correct casket. In order to win Portia's hand in marriage, her suitor must read several riddles and choose from three caskets made out of three different materials. In one of the three caskets is a portrait of Portia, which indicates that the suitor has chosen the correct casket. Not only is Portia upset that she cannot choose the man she wants to marry, but she is also depressed about her current suitors. As Nerissa proceeds to describe Portia's suitors, Portia highlights their various undesirable character traits and is disturbed that one of them might become her future husband. Overall, Portia is upset because she is powerless to marry who she wants and does not have positive feelings about any of her current suitors.