Referencing dialogue, how is Antonio represented as a melancholic character?

Antonio is the title character in Shakespeare’s The Merchant of Venice, and from his very first appearance on stage, he is portrayed as a depressed individual.
In fact, the opening lines of the play introduce Antonio's melancholy during a conversation with Salarino and Salanio:

It wearies me; you say it wearies you;
But how I caught it, found it, or came by it,
What stuff 'tis made of, whereof it is born,
I am to learn. (1.1.2-5)

In this quote, Antonio expresses frustration that he is unsure of the root cause of his depression. This establishes him as a moody character who might also be unsure of himself.
This characterization continues later in scene 1, when Antonio speaks with Gratiano. Antonio says that the world is

A stage where every man must play a part,
And mine a sad one. (1.1.83-84)

This line expresses Antonio's belief that it is his destiny in life to be consumed with melancholy; Antonio thinks that people are ruled by fate and therefore powerless to change their inner nature. It seems that Antonio likely won’t change by the end of the play.
Even after nearly sacrificing his life for Bassanio, Antonio is still the same melancholy man as he was in the very first scene of the play. When Portia and Nerissa are angry at their husbands for giving away their wedding rings, Antonio remarks,

I am th' unhappy subject of these quarrels. (5.1.238)

Although one could argue that Antonio is referring to the quarrels as unhappy, since the couples are arguing, the placing of unhappy right before the subject—which is Antonio—suggests he is talking about himself. Once again, Antonio proves himself to be a depressed person regardless of what has transpired in the play.