As I understand your issue, I suffer the same problem. I had that issue previously with some guns but it is much worse after badly bruising my middle finger middle knuckle during a slip-up with a grease gun (as in bearing grease). I'm OK with my Shield 9 but my .45ACP XDs is nearly unshootable for me at the moment. The XDs is more undercut behind the trigger guard allowing my knuckle to get behind it a bit while the shield isn't really undercut at all. Also my 3" 629 is nearly unshootable for the same reason while my 2.75" 69 is no problem. The 69 has the rubber Hogue Bantam Boot grips that hold my knuckle down a little lower so my knuckle doesn't get hit by the back of the trigger guard on firing.
I realize that the undercut trigger guard is generally considered an advantage that encourages a better grip. However, I've discovered it isn't always a nice feature. I'll need to find different grips for the 629, although I do like smaller more concealable grips. I'm not sure how to fix the XDs or the OP Smith. I'd kinda hate to glue something in to fill the gap.
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Well, having typed all of that I decided to look at the OP's gun in question. It doesn't seem to have the undercut trigger guard so what I just typed is, I guess, irrelevant. If you just read all of that I'm sorry but it took too long typing it to delete it.