None of us can say too much without more info (which paradoxically wouldn't help b/c we'd all "tl;dr") but let me ask this: if you were to quit, what is the chance they would succeed without you?

I also think you messed up by doing the hard work up front and then asking for equity. As it sounds to me, you were paid for the work you did therefore you get no equity; you were "hired" as you put it with a competitive salary for your position/experience so I don't see why you feel you deserve any equity TBH. If you get no equity, then mark it as a lesson learned and go work for another company out of school or your other startup.

Just an outsider looking in though. Good luck :)