Faith in the Real Presence is a gift, as all faith is. However, we also must do our part in working with the gift: being open to it and ultimately accepting it, even though we cannot fully see or understand what this gift is.
Think about the Gospel of John, Chapter 6. Jesus tells the crowd that "the bread which I shall give for the life of the world is my flesh" (John 6:51), but after some explanation, many of Jesus' disciples respond "This is a hard saying; who can listen to it?"(John 6:60). This teaching caused many of the disciples to stop following Him: "After this many of his disciples drew back and no longer went about with him" (John 6:66). Jesus then turns to His most trusted disciples, the Twelve. He asks them: "Will you also go away?" But Peter responds for them: "Lord, to whom shall we go? You have the words of eternal life; and we have believed, and have come to know, that you are the Holy One of God." (John 6:67-69).
Peter and the rest of the Twelve did not understand what Jesus was telling them any more than those disciples who left Jesus. However, what Peter and the Twelve did understand better than those other disciples was who Jesus was and the reliability of His word. Whereas the disciples who left sought to use understanding as the criterion for whether or not they would follow Jesus, Peter and the Twelve instead based their decision to follow Jesus on faith. Not that faith and reason are opposed. They are not. Faith without reason allows one to be easily led astray. However, reason without faith causes one to believe only what one's senses tells him or her.
St. Cyprian of Carthage wrote a treatise called The Death-Rate (ca. 252-253). Although he wrote this work in response to a plague and used the occasion to get Christians to think about their attitudes towards death, the passage can help us understand the attitude we should take towards the Eucharist, especially in light of John 6:
If a staid and praiseworthy man should promise you something you would have faith in his promise, and you would not believe that you would be cheated or deceived by one whom you knew to be steadfast in his words and deeds. God is speaking to you, and do you waiver faithlessly with your unbelieving mind? God promises that when you leave this world you shall have immortality in life eternal, and do you doubt? This is to know God not at all. This is to offend Christ, the Teacher of belief by the sin of disbelief. For one established in the Church, this is not to have faith in the house of faith. (The Faith of the Early Fathers by William A. Jurgens, Vol. I, #562, p. 224).
Peter and the Twelve (except for Judas Iscariot) understood that Jesus was "steadfast in his words and deeds." To doubt that what Jesus promises is true "is to know God not at all."
Later, St. Anselm of Canterbury (ca. 1033-1109), building on writings of St. Augustine, stressed the importance of faith as foundational to understanding. Anselm wrote, "Neque enim quaero intelligere ut credam, sed credo ut intelligam. Nam et hoc credo, quia, nisi credidero, non intelligam." ("Nor do I seek to understand that I may believe, but I believe that I may understand. For this, too, I believe, that, unless I first believe, I shall not understand.") If we are open to faith, we come to understand certain things that are not available to us outside of faith.
We will never understand the Eucharist this side of Heaven. It is indeed a hard saying. However, we must trust the One saying it, for He is steadfast in His words and deeds.