That's a fair question, and I do not think it will be easy. Our current political system makes it difficult to get any progressive laws passed, one of the chief reasons being that the government is skewed to favor less populated, i.e. rural, districts. It doesn't help to give wealthy corporations and individuals even greater ability to buy elections and politicians through secret, unlimited campaign contributions. The electoral college system and Senate make it practically impossible nowadays for progressives and liberals that do get elected to move legislation forward. Nevertheless, we have made progress over the long term, and I think we can do it again. More massacres of this sort are almost inevitable, so the problem won't go away when all the hubbub dies down. Younger people are getting less and less infatuated with gun ownership, and that is a good trend.
Look at it this way. Not so long ago, it seemed impossible that our country would elect an African American to the presidency, and same sex marriage seemed out of the question in politics. Things change, but we have to work to make them change.