Getting over the messiah complex

I have a confession to make: I want to fix everything.

Back when I was a Christian, I never really had to worry about injustice. The suffering we endured in our life here on earth wasn't all that meaningful, since we were guaranteed a happy and peaceful afterlife for our faithfulness. I figured that God would make everything work out for the good in the end.

Now, as an atheist, I have no such assurances. I look around at the amount of suffering in the world and I feel simultaneously motivated to do whatever I can to reduce it and impotent to actually accomplish anything meaningful in response to it.

Last Thanksgiving, I spent a week with my parents down in Mexico. On my second to last day there, while we sat comfortably in our condo watching people pass us by on the road, a homeless woman came along with her two children. While my parents commented about how amazing it was that she seemed so comfortable carrying her infant on her back in a sling fashioned from a scarf, I was watching her collect soda cans from trash cans along the street. All I could think of was that this woman was, because of purely random and meaningless circumstance, resigned to a life of struggle and hardship, scraping along with whatever she could manage just to be able to live to see another day. I desperately wanted to figure out what I could do to help her, and as soon as I started to think about it I realized that there must be tens of millions of people just like her all over the planet.

From my perspective as a person living in a first-world nation, this seems to be something that disappears all too quickly from view. There's no reason that I should have a better life than these people, and I shudder to think of all the potentially brilliant minds that fade into the background of the story of humanity simply because they're not wealthy enough to reach their potential.

So... what? Am I crazy to feel guilty about this? Is it wrong for me to willfully ignore these people so that I can get on with my life? What can I actually do to help fix the underlying roots of their suffering? Should I even worry about it?