How can I make my penis slimmer?

Hi, it's not so much your penis is to thick as your girlfriend isn't use to it, a vagina can stretch to accommodate a baby traveling through it and your penis isn't going to be that big!

Is your girlfriend a virgin, do you have plenty of oral sex first with her, does she get wet enough before you try to insert your penis, is your penis wet either from rubbing it over her labia and getting it wet from her secretions or using lubricants and is she fully ready for intercourse and relaxed because it sounds like she is tensing her vagina muscles as you go to enter her causing her pain and discomfort.