Staying motivated is simple: don't work on boring shit.

That may sound like a shallow phrase, but I've been there. You've convinced yourself that you're working on the most awesome thing in the world. You keep thinking that you're supposed to be motivated, because what you're doing is great. But deep down, you know that you'd rather do something else.

At that point it usually boils down to rewards. Is the reward worth continuing?

If the answer is yes, go on. Shouldn't be a problem. If the answer is no, you know why you aren't motivated. Drop it, look for something else.