I agree with Ray.
Trancada, if you are not fit enough to continue after the first leg, "to the extent that the flight may be endangered", you are supposed to "not perform duties on an aircraft" and take a crew rest.
That's your responsibility!
If you can't handle the responsibility that comes with the job it might be time for a career change.
But first try some "Controlled Rest on Flight Deck" when you feel tired.