You bought BRI-LAX cheap Now you want FCO-LAX which is not cheap. Your ticket was presumably non-refundable and non-reroutable. If you wanted to change, you should have purchased a more-expensive refundable/changeable ticket. It's the way it's been for years. If you could travel back 20/30+ years when tickets were still paper coupons, you could have done what you want to do at no cost and the airline(s) would be none the wiser until you were safely home.

No use complaining here. You are not going to get much sympathy.