I agree you need to talk to her find out why she is bitter.
I think bitterness can be overcome when you accept that life happens for a reason and whatever we are given in this life is destined for us good or bad. So there is no point in being angry or bitter about bad luck. Just accept it and learn from it.
I think you can stop showing too much love but be there to talk to her more and not emphasize love so much. Just listen to her. Go for walks every night. It's good exercise and a great way to talk.