If that occurs, I must adjust my expectations realistically. Not having realistic goals and ambitions can be a source of much frustration and grief. This can be difficult to do, though, because it may entail giving up on some of your dreams which can be hard to do. It's also something you don't want to do too easily. Dreams shouldn't die easily.
That said, becoming a mature person sometimes requires us to sometimes admit our dreams are outside of our reach. At those times we have to make adjustments and find another way to find fulfillment. The great thing is once we come to that conclusion and take our eyes off the unreachable goal, we are free to see all the other possibilities we have been ignoring!