Responding to disappointment in life is normal and fine, but what you choose to do and say in the given situation reflects a lot on where you have been and where you are going in life.
In Mark 9:14-29 there is an event with plenty of disappointment to share. There is a father who has a demon-possessed son that he brings to the Disciples for healing. The Disciples cannot grant the boy's healing, disappointment settles in for BOTH the father and the Disciples and arguing ensues. When Jesus arrives on the scene the father explains the situation to Him. Now it is Christ's turn to get frustrated and disappointed - He said,
When Jesus receives the boy He asks, "How long has he been like this?" The father answers, "From childhood" - giving the impression that his son is past this stage and could be anywhere from adolescent or teenage forward. So, though merely speculation, this father could have been looking for answers for his son for quite some time at this point. It is no wonder the father's disappointment carried over from the Disciples to Jesus - the father said to Jesus, "But if you can do anything, take pity on us and help us."
Jesus replies again, perhaps with still a little disappointed tone in His voice and says, "'If you can'? Everything is possible for one who believes." Notice in both instances in which Jesus responded that He allowed for His disappointment to be vented and known, but immediately He brought hope, He brought the solution - Faith. In fact, Christ brought to this situation Faith, Hope and Love. Faith for how we are supposed to be operating. Hope for how we are to operate going forward. And Love for how we are to minister in each case.
In the first instance, Jesus names the problem as an "unbelieving generation". Then He shows His frustration and disappointment by saying, "...how long shall I stay with you? How long shall I put up with you?" But finally He shows His compassionate side, full of grace, mercy and healing when He said, "Bring the boy to me."
In the second instance, you can hear frustration and disappointment in Jesus' voice as He says, "'If you can'?" - mocking the father's question on ability. Then Jesus addresses the problem of belief once again - "Everything is possible for one who believes." Finally, Christ's compassion is displayed in what could only be done through prayer and fasting in the first place, Jesus rebuked the impure spirit, saying, “You deaf and mute spirit, I command you, come out of him and never enter him again.”
When approaching the promises of God we may need to cry out like the father did - "I do believe, help me overcome my unbelief." However, following the example of Jesus, when our disappointments in life come, we need to have our Faith and Deeds ready to meet the real issue and Dispel The Disappointment.