Isaiah does not always make a clear distinction between the personal "I" that refers to Isaiah, and the prophetic "I" that couches the desires and words of God. In this instance, we have the prophetic "I" as the events indicated describe the care and working of God with his people.
The referent for "the former things" can either be the specific introduction where he is speaking to an Israel in need of repentance, or of a generic set of previous prophecies. Either interpretation fits and does no harm to the sense of the verse. The lord states two important facets of prophecy; first that it comes from the Lord, and second that they are fulfilled. I suspect that the "I did show them suddenly" refers to the fact that we are unaware of the fulfillment of prophecy until after the fact. Regardless of the prophecy, we are somewhat unprepared at its coming in that we do not necessarily see its advent clearly. When it arrives, it is "suddenly" for however long the prophecy has existed.
Application to Nephi's audience: This is a particularly powerful point that hits home not only to the audience for which Isaiah intended it, but to Nephi's immediate audience as well. This passage sets up the prophecy/fulfillment paradigm where the Lord indicates his willingness to guide his children, and then to make good on his words. In the Lehite experiences, there have already been multiple times where prophecy and fulfillment have played a critical role in their arrival in the New World.