Right. New Testament prophecies can fail. Old Testament Prophets, who wrote the Word of God, their prophecies are the Word of God and have not and will not ever fail. The prophecies of small “p” prophets of the Church age, are not the Word of God and can fail. (Though this is not an excuse for outright false prophecy, which is always born of false teaching.)
Paul is talking about the New Testament gift of prophecy when he says “prophecies will fail”.
Jesus is talking about the entirety of the Bible when He said “heaven and earth shall pass away, but the Word is forever “or eternal.
Just to go all metaphysical on you, the Word is the Logos, i.e. the meaning behind the actual language of the Bible no matter what different language it is translated.
This Logos is God.
In the beginning was the Word, and the Word was with God and the Word was God... and the Word was made flesh and dwelt among us.”
This Word or Logos is God. This is why it is Eternal.
And to add to your definition of “prophecy”; it says in Revelation that “the testimony of Jesus is spirit of prophecy”. This means that as we stand in our testimony of Christ, we naturally are walking in a prophetic mode, same as Jesus. Cool huh?
A definition given right in scripture and widely ignored within the church.
Just like with faith and people debating what it is when God tells us in Hebrews 11.