"Ye yourselves bear me witness, that I said, I am not the Christ, but that I am sent before him. He that hath the bride is the bridegroom: but the friend of the bridegroom, which standeth and heareth him, rejoiceth greatly because of the bridegroom’s voice: this my joy therefore is fulfilled." (John 3:28-29)

Consider here the humility of John.

Not only does he refuse to be called the bridegroom, he does not even number himself with the bride!

I do not believe that John felt he wasn't a child of God. I think he is simply saying that, even if he was just a friend of the bridegroom (and not the bride), he would rejoice that God is being glorified!

He felt that even if he existed only to serve everyone else with no benefit to himself, he would still have joy.

I think the writer of this gospel remembered that testimony when he was an old man. He wrote;

"I have no greater joy than to hear that my children walk in truth." (3 John 1:4)