Isaac trembled violently and said, “Who was it, then, that hunted game and brought it to me? I ate it just before you came and I blessed him—and indeed he will be blessed!”
When Esau heard his father’s words, he burst out with a loud and bitter cry and said to his father, “Bless me—me too, my father!”
But he said, “Your brother came deceitfully and took your blessing.”
Esau said, “Isn’t he rightly named Jacob? He has deceived me these two times: He took my birthright, and now he’s taken my blessing!” Then he asked, “Haven’t you reserved any blessing for me?”
Isaac answered Esau, “I have made him lord over you and have made all his relatives his servants, and I have sustained him with grain and new wine. So what can I possibly do for you, my son?”
Esau said to his father, “Do you have only one blessing, my father? Bless me too, my father!” Then Esau wept aloud. Genesis 27:33-38

It seems that after mistakenly blessing Jacob that Isaac could have just said, "Whoops" and blessed Esau the firstborn instead.  This was not the case.

When someone gave their word one time, they meant it.  Spoken promises and covenants meant more then before the days of broken contracts and litigation lawyers today.

Without written contracts, verbal contracts sealed the deal.  Isaac had just finalized his legacy and property rights to Isaac.  No way to rip that contract up.

God intended for this to happen.  While Isaac was a deceiver, Esau was a wild man, with no intention of following God.  Isaac was speaking for God and couldn't change the prophecy that the older would serve the younger as God told Rebekah in Genesis 25:23.  Isaac may have remembered that the moment he realized the deception.  "Wow, God was right!"

In Genesis 28, Isaac called Jacob in and gave him a proper blessing.