If you find yourself lethargic and unable to find the strength or will to motivate yourself, then try breaking your goal down into smaller, achievable tasks that work towards an endpoint. Don't indulge yourself or wallow in your situation, no matter how miserable you might feel. Do something else, such as going for a brisk walk, and settle to your task later. Sometimes a little break can clear your head and make you come back with more energy and motivation. Low motivation can sometimes be linked to conditions like depression, especially if you have other symptoms like poor self-image. Seek professional help if that applies to you and don't suffer in silence.