I have a couple thoughts on the matter. First, I’m sure it was heart wrenching for Abraham because (we can only assume) he loved his son very much. However, he also loved God. Not only did he love God, he trusted Him. He trusted God even though he surely did not understand why God was asking Him to do this. He could not have known about all the “prefigurement” that we talk about today. He only knew that God had given Him a command and he must be obedient to it. Abraham is known as the father of our faith, but remember that faith is a gift from God. So God gave Abraham the faith to carry out this terrible task. The key is that Abraham embraced that gift of faith where many of us do not. Scripture says “I can do all things through Him who strengthens me”. This was as true in Abraham’s time as in our own. God may set a terrible task before us, but if we turn to Him, He will give us what we need to get though it. Abraham certainly benefitted from an enormous amount of grace with the accepting the terrible choice to obey God and sacrifice his son. As someone already mentioned, this probably was not a foreign idea to Abraham. God was simply using the cultural ideas of the time to accomplish His work.
Remember too that everything God had told Abraham would happen did happen–even when they thought it would be impossible that Abraham would even have a son in his old age. God had already told him that it was through Isaac would be the one through which the descendants would bear his name (Gen 21:12). So, Abraham might well have been going up on that mountain trusting God’s promises so much that he did not have the anguish that you might think. In other words, Abraham knew that Isaac was the child of the promise, but that he had not yet had any descendants. How was God’s promise for Isaac to be fulfilled if Isaac were dead? Perhaps Abraham thought that if he were to go through with the sacrifice that God would have raised Isaac from the dead so that the promise could be fulfilled. (Now that would have been a prefigurement!!!) I obviously don’t know what he was actually thinking, but it is not outside the realm of possibility that Abraham trusted God’s promise for Isaac so much that he knew in his heart Isaac would not die or at least would not remain dead.