Can taking a deep breathe to calm yourself actually make you more nervous?

Only if you're under water.

its trying to stay cool under the panic which is the key.

A deep breathe to calm yourself is done through your core (middle/belly), not your chest. Your chest should barely rise at all. Slowly, slowly in through the nose and slowly out through your mouth. Simply taking a deep breath quickly through your chest isn't going to accomplish anything.

Pool. I think you need to find BIGGER things to get into panic fits about. You clearly wouldn’t be fit for a tournament if you can’t bypass the fear and keep calm.

It isn't the deep breath, it is the "letting it out slooooooooowly".

Breathing in deeply and out straight away will increase the oxygen in your blood and could result in your heart beating faster, instead of slower, which is your objective.