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