If you’ve started something, at least finish it.
Sometimes it’s all you can do. Sometimes it’s all you should do. But it’s important you do. If not for making it easier the next time around, for getting that all important sense of accomplishment.
When you’re really not feeling it, push your focus towards finishing first. Focus on getting over the line no matter how bad it looks. This will have you more than ready for when the next chance rolls around.