Forward pitch allows you to relax your hand in the ball more, which will allow you to loosen up your armswing and be more accurate. It sounds to me like you're still knuckling the ball (bending your thumb forward while in the thumb hole). Try keeping your thumb nail pressed against the back of your thumb hole throughout your swing.
The location of the callous also tells me you're turning your hand early- while it's still in the ball. Keep your hand behind it until your thumb clears the hole, and your callous will shrink.
As we get older, our hands lose strength and flexibility, so to combat this, we use forward pitch in the thumb hole. I'm at about 1/8 reverse, but my span is about 4 3/4 and 4 7/8, so my next move will probably be to cut about 1/8 - 1/4 off the span and go to about 1/8 forward. I hate to redrill a dozen balls like this, so I'll have to experiment with one first. It's what I did when I went from 5/8 reverse down to 1/8. Drilled a ball with 3/8 reverse, then one with 1/8. Then after a game or two, had all my stuff redone.
Experience is something you get immediately after you need it most
Bowl up a Storm!