Several years ago I bought a Mossberg 590, and the length of pull was a little longer than I expected, so i bought a Choate armor/ youth stock to compensate for the length of pull.  Unfortunately the geometry of the tang is the same on the shorter stock as a long stock, and my stubby fingers can barely engage the trigger unless I release my hold on the tang of the stock.  This makes it awkward trying to do follow up shots when trying to be semi-accurate with slugs.

Now that has been established, I would like to try a stock with a pistol grip, but I'm concerned by looking at some designs that the space between the grip and trigger may be too long or certain brands' grips.  Is anyone familiar with a pistol grip stock  that has similar feel and geometry to that of the Ar-15?  Some designs appear to have a built-in spacer between the grip and trigger.

My Benelli M4 could be considered somewhat similar to an AR regarding the grip & trigger orientation. If I were you, I would go to a big gun shop that has lots of stock. Hold different versions and see if they work for you. If they do then most similar configurations should work too.

Have you tried the Magpul SGA stock? I can't imagine anyone having trouble reaching or engaging the trigger. The LOP is adjustable, but not sure whether it is short enough for you or not. It's something to try out to find out.

Mesa Tactical

The issue is caused by the Mossberg's trigger placement on its receiver. You'll notice that there is a looooong, straight section of the receiver behind the trigger, which places the curved, semi-pistol grip section of the stock too far away from the trigger. This results in an excessive trigger reach when gripping the stock normally, at least for some people.

You can move your firing hand forward, holding half on the stock, half on the rear of the receiver. This is awkward, to say tthe least. You can get a pistol-grip stock. This reduces the trigger reach, because on the Mossberg PG stocks, the front part of the grip actually sits underneath a portion of that looooong rear receiver section. Unfortunately, the safety will be difficult to manipulate quickly on a Mossberg with a PG stock. Or, you can get a different shotgun; most, like the Remington 870, place the trigger at the rear of the receiver, so that the stock and trigger interface are anatomically-correct for most people.

I am not sure if your youth stock has the same dimensions as mine, but maybe the Magpul shotgun stock might be worth a look.

Pictured here (sorry for potato quality) is the Mossberg factory youth bantam stock. It's 12.5" LOP:


From the tang to the trigger is about 3 inches.



This is the Magpul Mossberg stock with 2 spacers, giving it a slightly longer LOP than the bantam at about 12.75":



From the tang to the trigger is about 2 inches.

That Magpul stock is probably the best bet before suggesting  Remington or something else with a shorter receiver to trigger length.

OP, I was the same as you. And I just overall never liked shooting shotguns. They don't fit me.
The stock, child & Magpull all had the same problems for me.  I didn't like the AR type stocks.  
I did find 1 I liked. The Knoxx Spec Ops.  Unfortunately the models that were good are not made any longer. However I have seen them pop up on Ebay. That stock worked well. But again overall shotguns were not for me.