For me this is usually caused by a major failure. The lethargy is what happens when one part of me knows something is very wrong, but the other part of me wants

For me this is usually caused by a major failure. The lethargy is what happens when one part of me knows something is very wrong, but the other part of me wants to continue going on like before anyways. What I do is find something I actually want to do and do that instead. You don't have to forget about the earlier goal; it becomes a puzzle that you need more clues for by doing different stuff for a while. In short, follow your heart and it'll all work out in time.