As you are a new shooter then I would also advise that you ignore all the "your gun is useless and doesn't fit" comments regardless of who told you, and go and shoot your new beretta and get your mount consistent.
Not sure how you can have a gun altered to a near perfect fit, when you haven't had the experience to mount the gun in the same way every time?
when I first started, I spent a good 10-20 mins every day just mounting my gun in the mirror to make sure I was looking down the rib straight.
Now, I'm left handed and know that I'm left eye dominant (I always pick up and look through a camera with my left eye, feels wrong with my right!) which gave me a good starting point of course, but I shot right hand cast guns for ages. In fact even my current krieghoff is technically a right hander.
But if you really want some more professional advice then I would go with what Ed has said, get your eye dominance sorted first. You might find that you'd be better off learning to shoot off your "wrong" shoulder to match your eye dominance which will feel strange but better to do whilst still a novice.
Just my penny's worth anyway.
Enjoy your shooting (that's why we do it after all!).