You should not accept it as a sunk cost, unless you are absolutely certain that meeting your goals is absolutely outside your power. (What makes a sunk cost sunk is the fact that it's too late to avoid it.) If you are certain of that, then I think you should change your goals.

(Instead of not accepting it as a sunk cost, you could take it as such and then treat meeting your goals as something that earns you a reward. That doesn't seem particularly demotivating, though.)