My knee jerk reaction to what would happen if God didn’t exist is that there would be no meaning. But notice: if I were to pick up a book written in gibberish, I could say it had no meaning only because I know what a book is supposed to say and how it’s supposed to be structured. In other words, no meaning pre-supposes meaning.
Hell can only exist because heaven exists. I do not mean that heaven can only exist because a hell does: moldy bread can exist only because bread exists but bread’s existence is independent of mold.
So to imagine a reality without God is worse than no meaning, it is no meaning to even be able to say no meaning. Hell is away from God but yet God still exists. Hell is a mercy to have been created because the no-God-alternative is even worse.
And no, meaning cannot come from within. It sounds good, and so does this bridge I made up and is worth what I say it is, not what you are willing to pay for it. And even then, I am not the bridge itself assigning its own value. So the counterexample is its own counterexample.