You are a good workman of God's word in that you readily admit, "I'm not sure." Lot's of things I'm not sure of either. Not to disparage anyone, but I think many Christians have a hard time admitting they are not sure or they don't know something.
Matt 3:11,
I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and [with] fire:
Luke 3:16,
John answered, saying unto [them] all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire:
The word "worthy" in both verses is the Greek word "hikanos" which means being "competent" of "fit" to do something. According to these two verses then, John did not feel competent or fit to baptize Jesus. That much we can know for sure. Unless there are other verses somewhere that elaborate on why he didn't feel competent to baptize Jesus, we should just leave it at that and not speculate or guess.
Perhaps there are other verses that elaborate on John's thinking, but I'm not aware of them. Do you know of any?
God bless.