First, define the task:

• What's the endpoint? (i.e. how you know when you're done?) (hint: the answer isn't 'when I'm physically and mentally exhausted')

• How much time are you going to spend on it right now? (roughly, and maybe set a timer/reminder if needed) (hint: the answer isn't 'until it's done, with no breaks, and ignoring my own basic needs and comfort')

Then break the task down into three degrees of difficulty:

1. The smallest, simplest step you could do, with ease and minimal effort, towards the endpoint that would involve some progress (i.e. make it easier to eventually reach the desired outcome)
2. A reasonable expectation of what you could easily achieve with some reasonable degree of effort
3. How you would go further if you get both of the above get done early/easily

Then just do what you can, and adjust your expectations in future.