Abraham fell facedown; he laughed and said to himself, “Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?”Genesis 17:17
God keeps repeating the same promises even though it's looking increasingly like that it won't happen. But if God says He is going to do something, He will do it.
When I have to wait, sometimes I have faith and trust God's word, sometimes I doubt things will ever change. Usually I oscillate between those two multiple times per day.
When I find myself doubting, I need to turn to God's word. The solution to doubt isn't to persuade myself to believe his word, it's to let his word work on me. Nothing is impossible for God. His promises are unwavering.