Sorry, not meaning to be, but I do appreciate the Shawshank Redemption reference!
I'm not sure a lot of people even think about this. Like you, they think "it just is." But WHY? You say it's collective wisdom coming from years of past civilization. OK, what about the first people (and I mean the VERY first people on Earth), where did they get their "common sense" or "sense of moral decency" from? They had no years of past civilization to rely on, so it couldn't have come from that, right? They WERE civilization at that point! If, as you say, it is not based on religion, then it is "based" on something - it has to be. You say your morals "come from within" - who put them within you? If nobody knows absolute truth, and we're all just guessing, then why can't I say "I'm going to murder Bill today, b/c I feel like it", if nothing within me says I shouldn't do that? I think we can agree that there IS something within all of us that GIVES us the sense of right and wrong, of moral decency. You asked if I know what's right and wrong without God telling me? I personally say "No", and here's why:
Romans 2:14-15 "Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law, since they show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts now accusing, now even defending them."
Our "conscience", the "law written on our hearts", was placed there by God Himself, that's where it comes from, I believe.