Bitcoin transactions are irreversible.

So when I broadcast a txn to the network, with payment to a merchant, wouldn't the merchant receive the bitcoin that I have sent him even if there is no confirmation..!

How can I double spend the bitcoin (this time with a higher txn fee), since my public key would get debited and the merchant's public key would get credited as soon as I broadcasted my txn to the network..?

Please help to understand.