People do differ in their personalities, and it's true that some people seem to be pessimists most of their lives (just as others seem to be born with a sunny disposition and are always looking on the bright side).
I've sometimes wondered if Thomas, one of Jesus' 12 disciples, was a pessimist. On one occasion, the Bible says, Thomas mournfully said to the other disciples, "Let us also go (to Jerusalem), that we may die with him" (John 11:16). In reality, however, only Jesus would die there; the others (including Thomas) would be spared. Pessimists have a way of distorting reality and thinking that things must be worse than they are.
But God doesn't want us to be gloomy and pessimistic about everything, as if we must carry the weight of the world on our shoulders all the time. Instead, He wants us to be realists, seeing the world the way He sees it -- with both its sorrows and its joys. And we can become like this, as we learn to trust God for the future and put our lives into His hands. Jesus said, "Do not let your hearts be troubled. Trust in God; trust also in me" (John 14:1).
Encourage your aunt to put her life -- and her whole future -- into Christ's hands. Then urge her to commit her concerns and her worries to Him in prayer. God loves us, and once we realize this our hope and confidence in Him will grow stronger every day.