You cannot be productive every day. Fact (probably). There will be days when you MUST DO THE THING NOW but you just can’t bring yourself to do the thing.
It’s ok. The world won’t end if you push back your deadline. Sure, someone might get pissy, but at the end of the day you’ll do better work when you’re in a good frame of mind. Sometimes it seems impossible to say no, but practising with smaller jobs can help to make it easier in the long run.