While things between the two of you may get hot and heavy and there are feelings that you share which have led the two of you to believe that sex is the next course of action you both want to take, from a female perspective, I believe that your girlfriend isn't quite ready. She may not recognize that fact or if she does, she isn't willing to say so to you or she might not know how to say it to you. When a female is ready for intercourse there is a natural lubrication that takes place coming from the uterus enabling the penis to slide in. It may not always be an easy entry but that's what happens when the female is excited, and wants to be penetrated. It's not her fault, she just inexperienced and doesn't realize how to allow herself to be stimulated enough so that you can get inside of her. Also you may be too inexperienced to excite her in that way. It's not all her responsibility, (although don't get me wrong I'm not judging or blaming either of you) sex is a two way street. Don't pressure her. Let some time go by and practice foreplay in a slow and patient manner. Try some research on female sexuality and arousal. Seriously. It might be what the two of you need.
Five and a half inches is considered average?!!! OMG!!!!!!!!!I hope that's not while erect, otherwise if it is erect and five and a half, I believe that's really quite small! Eight and a half is medium-large.