The angel is speaking on behalf of God and acts in this world on behalf of God. The Hebrew for "angel" is "מַּלְאָךְ" and the etymology is as follows:

Hebrew "mal'akh" (מַלְאָךְ) is the standard Hebrew Bible word for "messenger", both human and divine, though it is less used for human messengers in Modern Hebrew[1] as the latter is usually denoted by the term shaliyah (שליח). In the King James Bible, the noun mal'akh is rendered "angel" 111 times, "messenger" 98 times, "ambassadors" 4 times.[2] The noun derives from the verbal consonantal root l-'-k (ל-א-ך), meaning specifically "to send with a message" and with time was substituted with more applicable sh-l-h.[3] In Biblical Hebrew this root is attested only in this noun and in the noun "Mel'akah" (מְלָאכָה), meaning "work", "occupation" or "craftsmanship".

The morphological structure of the word mal’akh suggests that it is the maqtal form of the root denoting the tool or the mean of performing it.[4] The term "Mal'akh" therefore simply means the one who is sent, often translated as "messenger" when applied to humans; for instance, "Mal'akh" is the root of the name of the prophet Malachi, whose name means "my messenger". In modern Hebrew, mal'akh is the general word for "angel"; it is also the word for "angel" in Arabic (malak ملاك), Aramaic and Ethiopic.

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Because of this, the angel is both the speaker here and the one who can speak for God. When the verse says

וַאֲנִי אֶהְיֶה-לָּהּ נְאֻם-יְהוָה, חוֹמַת אֵשׁ סָבִיב; וּלְכָבוֹד, אֶהְיֶה בְתוֹכָהּ

For I, saith the LORD, will be unto her a wall of fire round about, and I will be the glory in the midst of her

The angel is the "I" and the angel acts on behalf of God. God sends angels to do the Lord's work. The angel is speaking on behalf of God.

Please, your entire problem is that you do not understand the Hebrew grammar. It's really very straightforward.