God is a Promise-Maker and a Promise-Keeper. In Psalm 89:34, God says "I will not violate my covenant or alter what my lips have uttered" Imagine that - the Master of the Universe, Creator of all that is; Sovereign God who can do anything He wants to, instead chooses to be bound by His word - when God makes a promise, He will surely bring it to pass.
Yet, for each promise, there is a process - a time of preparation, a time of waiting, sometimes a time (or times) of challenges and trials. Abraham went through a process after the promise of Isaac was given - he went through a time of waiting. In Genesis 15:4, God promised Abraham that the son of his own body would inherit his estate - the promise was given. In Genesis 16, Sarah believing herself to be too old to give Abraham a child, brought Hagar to her husband and Ishmael was conceived. For Abraham, this may have satisfied the promise he received - Ishmael was a child of his body, but since God promised him a child through his wife in Genesis 18:10, there was obviously another child to come. Ishmael was not the child of promise.
Finally, in Genesis 21:1, years after the promise was given, Isaac was born. Abraham was 100 years old! God IS a Promise-Keeper.
Like Abraham, God may give us a promise that involves a time of waiting. Such times call for patience, continued prayer and steadfast hope. Everyone experiences discouragement, but our faith in the Promise-Keeper must not fail. Hebrews 6:12 says "That ye be not slothful, but followers of them who through faith and patience inherit the promises." (King James Version)
While we wait for the perfect fulfillment of all God has spoken concerning us, let us wait with faith and patience - so we may be counted among the inheritors of God's promises.