Doug Wilson sheds some helpful light:
A discouraged saint is thinking about his own condition—it is all about his own condition. A humbled saint is concerned for the dishonor done to God by his sin, and not primarily concerned with the trouble he himself is in as a result of it.
It is possible to be discouraged, but not humbled, as Cain was when he complained that his punishment was more than he could bear. The point of grief was what was happening to him. It is possible to be humbled, but not discouraged, as the prodigal son was when he decided to return home. In that case, his discouragement brought him to humility, and humility brought him home, willing to be received as a servant.
You can read the entire article here.