The founding fathers of america instituting it into the declaration of independance and the american constitution was a great moment for humanity.
so long as you weren't white lower class/black/asian/actually p much everyone but the then v small middle class and upwards (and even then flower you) etc
This is the point I wanted to make. The American constitution was a great moment for propertied white men. We shouldn't forget that it was the basis on which a genocide was carried out.
Liberalism isn't opposed to racism and sexism - it never has been - at its core it has always been racist and sexist. It is not difficult, for example, to see the racism inherent in liberal and radical feminisms.
Since liberalism is racist and sexist, anti-racism and anti-sexism are not the domains of liberalism. That doesn't mean there aren't lots of liberals who call themselves feminists; David Cameron, Ed Miliband and Nick Clegg are all examples of liberals who have self-identified as feminist, but you'd have to be pretty naive to accept that at face value (the idea that any man can call himself a feminist deserves at least to be questioned). A genuine movement against white and male supremacy is possible - and indeed present - but it's not liberal.
So... what do all these social justice warriors do now
I'm worried that there is going to be a backlash
You answered your own question.
From my own personal experience I'd say that backlash is happening already, Younger people are far more conservative and reactionary now than they were when I was younger. And often say things that I actually find appalling. Of course this is totally subjective but as an example the other day I was doing some group study work and we got sidetracked as you do, during the conversation one of the people I felt had similar views to me actually stated (I kid you not) that Jews and Jehovah s witnesses only had themselves to blame for their treatment under the third Reich because they failed to drop their beliefs and integrate. And there was somewhat of a generational gap between those of us who grew up in the 90's and those who did so in the 00's.
Perhaps I find stuff I am opposed to in order to push back (especially online) but there seems to be a resurgence of sickening opinions and the idea that racist/sexist comments are 'just voicing what everyone is thinking'.
its much easier to say those things when all you've ever known is living in a peaceful democracy. its only when fascism comes bombing at your door and murdering your loved ones that you realise why evil should be fought and opposed
Democracies aren't peaceful. We live in an intensely violent society where violence is an ordinary part of everyday life - so normal in fact we usually don't even notice it. Borders; prisons; police, these are some of the more obvious examples, but also work, state bureaucracies, schools, the family, property generally, are all institutions reproduced through violence (and the threat of violence). Democracies are in a constant state of conflict where power defends itself from rivals and the dispossessed, those without power. That confrontation is almost always violent. Where democracy fails to contain that conflict, it inevitably gives way to totalitarianism and fascism. Fascism doesn't come "bombing at your door," the powerful readily invite it when democracy fails to preserve their privilege; that's been the case in virtually every example of existing fascism you care to mention. Democracy leads to fascism.