I WAS SITTING on a train from D.C. to Philadelphia, coming from a Senate press conference about gun trafficking, when I read about yet another mass shooting - this one in San Bernardino, Calif. I'm reminded of a day almost exactly three years ago, when sitting on the same train coming from a gun-violence prevention conference, I read about a mass shooting at a school in Newtown, Conn. Predictably, both times, my email blew up and my Facebook feed was full of laments - "no more," "enough," "God, stop this please," "this has to stop." People are clearly tired, frustrated and angry.

And so am I. But here's what I know: "this" won't stop until we stop it. The truth is, this is on us.

Gun violence is not a city problem or a rural problem or a suburban problem. It's not defined by race. It's not a mental-health problem or a drug problem. It's not a Republican problem or a Democratic problem. It's not a Christian problem or a Muslim problem or a Jewish problem or an atheist problem. It's an American problem. And that means it's a problem that belongs to all of us.

America is average when it comes to non-gun crime compared to other developed countries. Our rates of mental illness are the same. We watch the same movies and play the same video games and listen to the same music. So why do we have so much more gun crime than any other developed nation? Why do we have nearly daily mass shootings in homes, businesses, places of worship, schools, etc.? Why do we have more suicides? Why do we read so often about accidental shootings by children and adults?

The answer is simple: It is terrifyingly easy to get a gun in America. We require background checks for some sales, but not all sales. We have legal avenues where you can buy a gun with no ID shown, no questions asked. If you were someone not legally allowed to buy guns, wouldn't you choose the option where you can avoid a background check?

Pennsylvania doesn't require any waiting period to buy a gun. There is no licensing or registration process. There is no mandatory training before purchasing a gun or getting a concealed carry license. We don't require people to report when guns are stolen. People on the terror watch list aren't banned from purchasing guns.

We even block the CDC from doing or funding real research about gun violence through outrageous appropriations riders.

So why are we so surprised that people who clearly shouldn't have guns so easily get them?More importantly, what are we going to do about it? How are we going to solve the problem?

The problem is not lack of knowledge or resources. It is lack of will and courage on the part of policymakers. So if we want change, we need to address that lack of will and courage with our voices, votes and dollars. Those are the currencies that matter most.

If you care about Columbine, Sandy Hook, Tucson, Aurora, Charleston, San Bernardino, Baltimore, Hatboro - if you care about gun violence and want to fight it, here's what you have to do:Call the governor and the president, write to your legislators and tell them you've had enough and that your future votes and dollars will be given in direct proportion to how they act to fight gun violence and solve this American problem. Let's solve it the American way, with the tools of democracy.

Prayers and tears are important, but action is how we honor the victims. It's up to us.

We have an American problem, and we need to solve it the American way.

Get calling, get writing, get on the bus to Harrisburg, just get going. We can't afford to wait anymore.

Shira Goodman is executive director of CeaseFirePA.