But what about the quantitative way? :(

Edit: Forget that... I finally get it. Like, really get it. You said:

and find it literally unbearable. All of a sudden, it's clear that to be a good person is to accept the weight of the world on your shoulders

Oh, my gosh... I think that's why I gave up Christianity. I wish I could say I gave it up because I wanted to believe what's true, but that's probably not true. Honestly, I probably gave it up because having the power to impact someone else's eternity through outreach or prayer, and sometimes not using that power, was literally unbearable for me. I considered it selfish to do anything that promoted mere earthly happiness when the Bible implied that outreach and prayer might impact someone's eternal soul.

And now I think that, personally, being raised Christian might have been an incredible blessing. Otherwise, I might have shared your outlook. But after 22 years of believing in eternal souls, actions with finite effects don't seem nearly as important as they probably would had I not come from the perspective that people's lives on earth are just specks, just one-infinitieth total existence.