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The reference to "the first and the last" is a conscious echo of Isaiah 41:4:
Isaiah 41:4: Who hath wrought and done it, calling the generations from the beginning? I the LORD, the first, and with the last; I am he.
Isaiah 41:4 is, in turn, part of a narrative about the Creator -- the first person of the Trinity -- as we see in Isaiah 40:28:
Isaiah 40:28: Hast thou not known? hast thou not heard, that the everlasting God, the LORD, the Creator of the ends of the earth, fainteth not, neither is weary? there is no searching of his understanding.
In verse 44:24 the narrative again reminds us that it is God, the creator of all things, who is talking:
Isaiah 44:24: Thus saith the LORD, thy redeemer, and he that formed thee from the womb, I am the LORD that maketh all things; that stretcheth forth the heavens alone; that spreadeth abroad the earth by myself;
The importance of "no God beside me" is not a denial of the Trinity but of paganism. Most biblical scholars regard Isaiah chapters 40-55 to have been written during the Babylonian Exile by an anonymous prophet now known as Second Isaiah to distinguish him from Isaiah, son of Amoz. It was during this period that the priesthood began to assertively declare that there is only one God.
Upvote:1
In grammar, "I" is a personal pronoun referring to oneself. It is a first-person singular. It's important to understand the time in which Isaiah was written and for whom it was written. We can only work with what theology Isaiah provides us, so be not tempted.
Whether or not you believe in the Trinity, it can be agreed that YHWH is the Father. (See Isa. 64:8) A trinitarian view of there being no god besides YHWH is that the three persons share the same substance, and there is only one substance, not three.
The trouble you seem to be encountering is that if YHWH is the Father, and if the Father says he is the only one who is god, i.e., only he possesses this divine nature, then where is the room for his word and spirit? I think Justin Martyr said it best:
For He can be called by all those names, since He ministers to the Father's will, and since He was begotten of the Father by an act of will; just as we see happening among ourselves: for when we give out some word, we beget the word; yet not by abscission, so as to lessen the word [which remains] in us, when we give it out: and just as we see also happening in the case of a fire, which is not lessened when it has kindled [another], but remains the same; and that which has been kindled by it likewise appears to exist by itself, not diminishing that from which it was kindled.
Chapter 61. Wisdom is begotten of the Father, as fire from fire