Your best will change form day to day, whether you slept well, are healthy, hydrated, etc, all effect how you do that day.
Your best will change. Keep doing your best.
As long as you give each challenge your best effort, than you can rest easy, no matter what the outcome. There is peace of mind that comes when you have given it your all. You won't have self-judgement or regret, wondering if you could have tried harder when things don't work out. And when things finally DO work out, you will have rocked it!!
This fourth agreement teaches us that when you stp up, you step AAALLLLL the way up, and do your best!!
And you'll be surprised how the Universe responds! ;)