Well, essentially it's your fault.
When you first turn the phone on, the Android software (this applies to any Android phone) creates a small file on the device that controls battery life.
Do to the fact you didn't charge it fully before you turned it on, the phone is convinced the maximum capacity of the battery is much less than it really is.
To fix this you need to "recalibrate" you battery.
What you need to do is charge the phone before it reaches less than 20% battery life for at least the next 5 charges. The reason you don't need to wait until 0% is that the One X uses Lithium Polymer, a much more efficient battery Tech in that respect.
Making sure the phone is turned off when you charge it, and that you let it complete the full charge. The light will turn green at this point.
After the 5th full charge while off, boot the phone. Once it's finished, hold down the power button until the phone says it will reset. Keep holding the button.
Once it reboots, you're done!
I've the same phone and get between 14 hours (of hardcore use with the brightness set to 50%)
or 40 hours if I'm keeping it in my pocket and not using it much expect for a few texts and calls.
To all the uneducated people who can't spell in these comments, I do recommend the iPhone 4S for you. It's made for people who don't understand technology, but like to throw away money.