It sux mate I feel for you. I am not sure if my ex has found someone else yet but I know he is online dating looking. Just for a rebound I suppose. I am like you, can't even entertain the thought of someone else right now but I think that is the best outcome to be honest. Gives us time to flush the ex out of our system. She is just looking for someone to boost her confidence and she is probably scared of being alone. It won't last.
And chances are she isn't over you, just transferring emotions or hoping through someone else she will forget about you. The oldest trick in the book. Hang in there.