I am Alpha and Omega These are the first and the last letters of the Greek alphabet, and denote properly the first and the last. So in <Revelation 22:13>, where the two expressions are united, "I am Alpha and Omega, the beginning and the end, the first and the last." So in <Revelation 1:17>, the speaker says of himself, "I am the first and the last." Among the Jewish rabbis it was common to use the first and the last letters of the Hebrew alphabet to denote the whole of anything, from beginning to end. Thus, it is said, "Adam transgressed the whole law, from ‘Aleph (’) to Taw (t)." "Abraham kept the whole law, from ‘Aleph (’) to Taw (t)." The language here is what would properly denote "eternity" in the being to whom it is applied, and could be used in reference to no one but the true God. It means that he is the beginning and the end of all things; that he was at the commencement, and will be at the close; and it is thus equivalent to saying that he has always existed, and that he will always exist. Compare <Isaiah 41:4>, "I the Lord, the first, and with the last"; <Isaiah 44:6>, "I am the first, and I am the last; and beside me there is no God"; <Isaiah 48:12>, "I am he; I am the first, I also am the last." There can be no doubt that the language here would be naturally understood as implying divinity, and it could be properly applied to no one but the true God. The obvious interpretation here would be to apply this to the Lord Jesus; for:
(a) it is he who is spoken of in the verses preceding, and
(b) there can be no doubt that the same language is applied to him in <Revelation 1:11>. As there is, however, a difference of reading in this place in the Greek text, and as it can. not be absolutely certain that the writer meant to refer to the Lord Jesus specifically here, this cannot be adduced with propriety as a proof-text to demonstrate his divinity. Many MSS., instead of "Lord," kurios (grk 2962), read "God," Theos (grk 2316) and this reading is adopted by Griesbach, Tittman, and Hahn, and is now regarded as the correct reading. There is no real incongruity in supposing, also, that the writer here meant to refer to God as such, since the introduction of a reference to him would not be inappropriate to his manifest design. Besides, a portion of the language used here, "which is, and was, and is to come," is what would more naturally suggest a reference to God as such, than to the Lord Jesus Christ. See <Revelation 1:4>. The object for which this passage referring to the "first and the last — to him who was, and is, and is to come," is introduced here evidently is, to show that as he was clothed with omnipotence, and would continue to exist through all ages to come as he had existed in all ages past, there could be no doubt about his ability to execute all which it is said he would execute. Saith the Lord Or, saith God, according to what is now regarded as the correct reading.ÿ Which is, and which was, … See the notes on <Revelation 1:4>. The Almighty An appellation often applied to God, meaning that he has all power, and used here to denote that he is able to accomplish what is disclosed in this book. (from Barnes’ Notes)
BTW its application to Christ rebuts your denial of his pre-existence