I think this is the best answer so far, if the things you used to love don't satisfy you the way they did, you're probably burnt out. If, on the other hand, your not meeting your own expectations stems from screwing up priorities (avoiding what you don't enjoy), you're likely just being lazy.
If I spend too long on a project that I just want to finish, I generally find myself looking at other more interesting projects and working a little on them.
Sometimes this is being lazy but sometimes you just need the change to get back on track with your mundane project.
If all you want to do are the things you like then you're probably being lazy.
Not fool proof - but a starting indicator.