About a year ago I picked up a bunch of new PX4s, 9mm Compact for wife to carry,

9mm Full size for me to carry, and a 45ACP Full Size just because the price was right.

Love the 9mm's but the 45ACP stings my trigger finger something awful.

Is this a defect in the pistol, or a design flaw, or something I just need to get used to?

Really like these pistols, hate the thought of getting rid of the 45ACP, but if it's no fun to shoot

then it can't stay in my gun safe.

I read about this in the 40 cal version of the PX4 many times over the years - I'd recommend going over to the Beretta Forum and looking up old threads. I don't think there is a fix if you are one of the unlucky ones