Actually, long lasting cold rain is pretty similar to a ride in very cold weather or at least can be in terms of solution.
You have to prevent water from coming in and sweat from coming out (contaminating the insulation of your gloves)
With your hands getting soaked and cold you have just discovered what the false waterproof claims are.
The only true waterproof gloves are made of rubber, PVC, latex etc.. the other are just water resistant/repellent so forget about PI, Gore, craft and other known cycling brands when you have to deals with long lasting cold rain and go for something like some household cleaning gloves for instance.
Your current gloves will fit the bill, just sandwiched them between 2 true waterproof membranes
some nitrile gloves under your current gloves and some motorcycle rubber gloves over your current gloves like those for instance:
http://www.motorcycle-superstore.com...in-Gloves.aspx