I used to do this a lot:
When I’m in the zone, I can finish a story or chapter in a couple of days.
Or, I can write about 6000 words in a day.
Then, the next day, I’ll look at my progress and say, “Wow! If I could do that every day, I’d finish this whole project in no time. That should be my goal from now on!”
Can you see where this is going?
Well, of course, I would have bad days where I didn’t get anywhere near my goal and try to make it all up the next time. Pretty soon, it would spiral so I wasn’t making any progress. I’d get discouraged, and I wouldn’t be writing anything.
This is not helpful! It’s self-sabotage!
Set your goals in the midrange of what you’re capable of. If you find you’re consistently exceeding a goal, then push it higher. It’s just like physical training. You don’t set a goal to max out your performance every day. If you do, you’ll hurt yourself! You set a goal to build endurance. My rule of thumb is at least one chapter per week or at least 2500 words per day. Usually, I do more than that, but on the days I don’t exceed my goals, I’ve still got a success under my belt.