I'm thinking of cancelling my joint birthday party
Why do you want to cancel your joint birthday party?
Failure on my party isn't an option, I can't risk it!
It's wrong to cancel a joint party at the last minute as this will negatively affect another person's plans. It's best to discuss the  birthday plans with the other party host.
It's hard to focus on what really needs to be done when you have to follow through with your plans and you will always be disappointed with your results.
I understand, trying to make anything perfect can be stressful. It's best to work together with the other party host to ensure that you are both happy with the party.