Well Steven, maybe I can help you out on this issue. The first thing you need to know is that "THE" angel of the Lord first appears at Genesis 16:7 as the angel of the Lord.
Secondly, "The" angel of the Lord is the pre-incarnate Jesus Christ and I can prove it from scripture in the Old Testament. I am not going to do it now.
Thirdly, there is only ONE angel of the Lord in scripture, and he never appears in the New Testament as "THE" angel of the Lord. He is mentioned in the New Testament by Stephen at Acts 7.
By now your probably wondering why I'm taking the time to say, "THE" angel of the Lord and why it is "capitalized?" You quoted Acts 12:7 where an angel struck Peter and woke him up.
There is a difference in the function between the word, "an/a" and the word "the." The chief grammatical function of "an" (or a) is to connote a thing not previously noted or recognized, while "the" connotes a thing previously noted or recognized.
At Acts 12:7 this is an ordinary angel who just appears and helps out Peter. At Acts 5:19, "But an angel of the Lord opened the gates etc. Same thing at Acts 8:26, "an angel." Matthew 1:20, 1:24, 2:13 and Matthew 28:2.
You mentioned the book of Revelation and "an/a" angel of the Lord does not appear. However, Jesus appears in the book of Revelation but not as "THE" angel of the Lord. I hope this helps!
If you want to learn about the angel of the Lord start at Genesis 16:7 and read the rest of the chapter taking note of vs10 and vs13. Also read Genesis 17:1-3 and put it together with Genesis 16:10 and the word, "multiply."
Also read Genesis 18, especially Genesis 22 and all of Exodus chapter 3, who's in the burning bush?