Some girls don't understand that it is hard for most boys to express their feelings. It is important that you explain to her that it is hard for you but you are willing to try if she gives him a chance. She has to realistic with this though - it is not just going to happen overnight for you. As well as sometimes feelings are expressed through actions and not always words but it is always nice to have a bit of both.
If this does not work then you must move on. There will be people that come in and out of our lives that we all can take something from and learn from - maybe this is your learning experience that you can leave this relationship with and when you are ready and are in a new one you will know that the expression of feelings are an important part of being together.