We are all accustomed to promises. We are also accustomed to seeing them made and broken. Anyone who has lived for a number of years would certainly never lay claim to having kept every promise made. There are many reasons why this is true. Sometimes we forget, sometimes we are negligent, and sometimes it may be due to circumstances beyond our control.
People do change their mind, and they do break their promises.
God's promise to Abraham was first spelled out in Genesis Chapter 12. It was repeated in Chapter 22. Now a promise is of no more value than is the ability of the one who makes it to carry through. It also includes willingness to do so. God did carry through with Abraham.