It is very possible that you are falling into a relationship. In fact, simply wondering about whether you are falling into a relationship is often an indication that you are.
A good way to be more expressive with your girlfriend is to learn to start trusting her and believing in yourself.
The reason why you may not be expressive could be because you are uncomfortable about something. You may think that if you tried to be more expressive in some particular way, she would not approve.
That may worry you. You may also think that you simply don’t know how to do it because you have never been expressive in the past.