I seldom share my feelings with my girlfriend
Why do you not share your feelings with your girlfriend?
I want to tell her more but I get fearful she won't understand and then dump me.
Being fearful is something that occupies your mind too much, but your girlfriend needs to know what you are feeling. Shutting down and being afraid doesn't set the stage for a happy romance, you should be able to tell a person what you feel for them at least. Many people don't share every little thing but still have a happy romantic life.
I don't want to lose her.
I understand that you're scared, but I think that if she truly loves you then she'll be happy to hear about your feelings.