Lo Bashamayyim He... But then what?

I just finished reading a fascinating post by XGH over at Modern Orthprax & Heterodox, asking what it means to say that Torah isn't the directly-dictated word of God (i.e., "Torah mi Sinai" or "TMS").  My comment is here.

I know it's naive of me to say/ask, but this question -- what is the meaning and purpose of the mitzvot and of Jewish practices and rituals more generally -- has bugged me for as many years as I can remember thinking about Judaism.  In some sense this is surprising:  Being a born and bred RJ, this just isn't a question that gets asked too much.  Of course the Torah is a human-authored work.  Of course God didn't tell us do these things, don't do these things, etc.  Of course the main question should be what has meaning to me rather than what does God want me to do.

But where does this leave us?

Don't get me wrong, I'm not going frum; for many reasons I'm perfectly happy to identify myself as a RJ.  For reals.  And yet...the more I look at certain things, the more unsatisfied I get.  It's not something that more observance or becoming a BT would help.  There's just too much Jewish stuff I can't agree with or be a part of no matter what explanations are given.  The dissatisfaction stems from following the logic:  Choice is important to me in terms of what I observe and what I don't observe; but once I start picking and choosing, why this and not that?  What if we all pick and choose this and not that?  Then what becomes the basis of what we are?  If, at the bottom, mitzvot are just socio-cultural artifacts, then it really doesn't matter which I do and which I don't.  But then what does it mean to be a Jew? 

This is the flip-side of the lo bashamayyim he story, at least as I understand it:  God says, in effect, I have given the Torah to human beings, and now I'm out of the interpretation business -- it's up to them.  Therefore, while Torah may no longer be in the heavens, it certainly came from the heavens.  But I can't believe this in any literal sense.  It didn't come from the heavens; it was written by (divinely?) inspired human beings trying to make sense of their place in the world.  The problem is that it's not really honest for me to accept lo bashamayyim he when I don't accept min hashamayyim ba, right?