Take a deep breath.
Have a cup of coffee.
Perhaps talk to a good friend.
Then go and take care of that child who depends on you totally for his/her psychological, emotional and physical well-being.
Parents all have tough times. We doubt ourselves. Our children challenge us.
But in the end we are all they have. And we love them. Deep down inside, somewhere in middle of your exhausted body, is the love for this child.
Do not, at any point, let this child doubt your love. It is a monstrously damaging thing to do. Muscle through whatever is bothering you. You are the parent.