Perhaps the porch is better lit than the dark bedroom and, upon returning to the bedroom, he couldn't see well enough to notice that his girlfriend was not longer in the bed.
Obviously in such a case you'd think he would make certain she was still there. And perhaps he'd even wake her. But, according to his story, he didn't.
It's also possible he felt an intruder was a looming threat and he grabbed his gun first. If he truly was "terrified" as he says, it's not that surprising he fixated on the threat.
In all, I think OP is admitting in his statement that he committed manslaughter. He was negligent and stupid and wound up killing his girlfriend.
The trial will be about whether his story is true, or whether the circumstances of her death are quite different (was there an argument, etc.). That will make the different between a premeditated murder and manslaughter (does that exist in SA?) or some lesser kind of negligent homicide charge.