Why can't I express exactly how I feel towards her?

Been chatting with this girl for over a year. We seem to really hit it off and can talk about anything, no matter how personal. At first I didn't think it would work out because of the distance but we have grown closer to each other. We text, chat, email, fb as much as we can. She is more open than I am and tells me she wants to move forward with me and loves me. She is always on my mind, I don't want what we have to ever end. I have feelings for her but can't express them when i want to. We have both been hurt in the past by our ex's but we have learned to move on and are both excited about what we have started. I have told her that my feelings are there but I have a hard time expressing myself and that I love what we have. Why can't I express myself more in words? She has been nothing more than patient and understanding with me. I don't want to lose her.