The difference between sunk-cost fallacy and persistence is that of motivation. If you keep doing something because "you've worked so hard already" then that's sunk-cost fallacy. If you keep doing something because "success is just around the corner" then that's persistence.

You can't go back in time and not work hard on something, so whether or not you should continue is purely a function of whether or not you think you will succeed, not a function of how much effort you've already put into it.