As I read it in John 18, John himself was known to the high priest.  They may even have been friends.  So John went right on into the courtyard with Jesus.  Seems as if the high priest was being careful as to just who got into the yard and could testify to the events there.  Also, they were so afraid of the people that they arrested Jesus at night.  They dared not take him in the day.  So they likely were trying to keep his presence at Annas' house pretty quiet.

Since Peter was not known to the high priest, they didn't let him into the courtyard.  But since John was inside, and well known, he was able to send word to a servant girl to open the gate for Peter.  She was being careful not to let anyone in who was loyal to Jesus.  So she asks Peter if he is a follower of Jesus, and Peter says no.  Looked at several other translations besides ESV.

17 Then the servant girl who kept the door said to Peter, "You are not also [one] of this Man's disciples, are you?" He said, "I am not." [Jhn 18:17 NKJV]

17 The woman asked Peter, "You're not one of that man's disciples, are you?" "No," he said, "I am not." [Jhn 18:17 NLT]

17 "You aren't one of this man's disciples too, are you?" she asked Peter. He replied, "I am not." [Jhn 18:17 NIV]

All make it clear that he would not have gotten inside if he had admitted to being a disciple.  Maybe this is how he justified denying Christ.  He wanted to get in and see what was happening.  He told himself he needed to be inside if he was going to be any help at all.  So he said no.

To tell yourself you are getting closer to someone by denying any association with them is a bit of a lapse in logic though.  To tell yourself you are better able to influence if you're part of the inner circle is likewise dangerous.  Think of the implications.  Peter purposely associated himself with the crowd in order to get in and see what was going to happen.  How many churches try to make themselves appear secular in order to reach a larger audience - to pack the pews?  Is this the same thing, or do I have it wrong?

Then this verse:
18 Now the servants and officers had made a charcoal fire, because it was cold, and they were standing and warming themselves. Peter also was with them, standing and warming himself. [Jhn 18:18 ESV]

This is phrased as if Peter was over by the fire with the bad guys, but John was not with them.  John was standing elsewhere.  Apart, in the cold, risking being recognized as a friend of the accused.  But Peter, having already claimed to be one of the accusers instead of a disciple of Jesus, is finding his new friends very accommodating.  They built a nice warm charcoal fire.  Heat without too much smoke.  And it's pretty dark with no flames, so he can be "in the group" but still a bit anonymous there in the dark.  He was getting accustomed to being warm.

After being questioned by Annas, Jesus is sent bound to the high priest Caiaphas.  As this occurs, Peter is still by the fire, and again he is questioned about his association with Jesus.  Again, in order to remain where he is, Peter denies any association with Jesus.  This time, he denies so he can stay by the fire where it's warm.  This time, his personal comfort is part of his reason for answering as he does.  To take "the other side" at this point would mean he had to give up warmth, acceptance, and to some degree, safety in the group.

But then an eye witness, a relative of Malchus, says he actually saw Peter with Jesus in the garden that very night.  Peter's third denial is the most incriminating yet.  He lies to an eye witness.  I'm sure they were pretty sure he was lying by this time.  His cover is blown at this point.  He lied to stay "in" and it backfires.  Now they step away from him, and as the rooster crows, Jesus looks at him across that charcoal fire.  At that moment, he was an utter failure at everything he had ever considered important.  He was devastated.

So he lied to get in, he lied to stay warm, and then he lied to stay hidden.

I think it is clear that inside where it mattered Peter WAS a disciple.  But like us, when the critical moment to confess Jesus before men came, he was weak and more concerned with the moment than with eternity.  But there is no indication at all that he stopped believing that Jesus Christ was the Son of God during these events.  He sinned, he failed, but he did not fall.