"He did not weaken in faith when he considered his own body to be already dead (since he was about a hundred year old) and also the deadness of Sarah's womb. He did not waver in unbelief at God's promise but was strengthened in his faith and gave glory to God, because he was fully convinced that what God had promised, He was also able to do." (Romans 4:19-21)
I think there is a school of thought out there that says, "Don't look at the obstacles; don't feed your doubts."
This is not what Abraham did, and it is not the course of genuine faith. True faith does not live in denial about the obstacles. It considers them--mulls them over--as Abraham did. But it does not stop at weighing the negatives. Faith then considers the power and faithfulness of God.
I think Abraham's logic may have gone something like this: "Let's see: God promised to give me a son and make me a great nation. But I'm one hundred years old. My body doesn't work like it used to. Sarah's way past child-bearing age. Hmmm, those are some heavy challenges. But, lets see, God is the one who made me this promise. He is able to raise the dead and give them life." He picks up a stick and begins to scratch some figures in the sand. "Obstacles: two. But God: one... with one zero behind it... and another zero... and another zero... and another.... Ah, forget this! I'll just go with God!"
Abraham did not deny the obstacles. Those were real. He looked them squarely in the eye! But then he moved on in his computations. He added "the God factor" into the equation. God smashes the competition!
And don't miss this: Abraham "gave glory to God." It helps to give expression to your faith. Something about confessing God's power and faithfulness that bolsters our faith.
The result was that Abraham believed God. His belief was counted for righteousness for him and the promise was fulfilled.
Don't deny the obstacles, but never stop there. Not only consider them, but consider Him!