Outshine your former self.
When your brain tells you that you’ve reached your limits, you actually have NOT — you still have a room for doing a bit more.
You can use that tiny room for anything. You can just complain how hard it is, think about quitting, or actually quit and do another thing. Or you can have your brain shut up and keep focusing on what you are doing.
Improvement often comes from doing a bit more. From going an extra mile. From challenging your former self. The outcome may not always be successful, but you will almost always get something meaningful from the experience. At least, you become a bit stronger than yesterday.