Yes, the world's needs are overwhelming, and no matter how much we do, there is always more to be done. During His ministry, Jesus healed many people but He didn't heal everyone.
But that shouldn't keep us from doing whatever we can, and trusting God to use it. Have you ever tried to put yourself in the shoes of someone in an impoverished country who has a devastating illness? How would you feel if an organization showed up with medicine that would cure you but they gave away their last dose just before it was your turn? With only a few more dollars, they might have been able to give the medicine to you also.
No, you can't do everything but you can do something. Do you remember the poor widow who gave two small coins to God's work? Others gave much, much more, but Jesus told His disciples, "I tell you the truth ... this poor widow has put in more than all the others. All these people gave their gifts out of their wealth, but she out of her poverty put in all she had to live on" (Luke 21:4).
Ask God to guide you in your giving and to show you where you should give (beginning with your church). Then pray for those who are working in difficult places in the name of Christ, that God would encourage them and use them to point others to Himself.