I’m sorry you haven’t had anyone to talk about your stutter with. I imagine that must be difficult. I’ve done the opposite and I think it really helped me. Communication is key in any relationship. Having a stutter is difficult. If you’ve barely ever had a conversation like this it’s difficult to cohesive thoughts. Honestly, I would recommend writing it down first. Understand how you felt and why you felt that way. Once you’re ready go and speak to your girlfriend.