I can't short hop reliably either, so don't worry, you aren't the only one
I had basically been told the same thing, just keep practicing it, and you will get it. This was a couple of months ago, and I still haven't gotten it. I do find that if I focus my attention specifically towards trying to short hop (at the expense of whatever else may be going on at the time), I do seem to be able to do it more frequently.
You only have a window of a few frames to release the button, and it's not the same for all characters! Some characters are literally twice as easy to short hop with than others. I thought I saw a chart once showing how many frames you have for each character, but maybe that was for a different game in the series.
The good news is, because Smash 4 doesn't have SHFFLing, the short hop is nowhere near as important as it was in melee. Though still an important move, you can get by without it for the most part if you aren't trying to be tournament-level good.
But yea, just keep trying to press it fast. Maybe you'll get faster.