I feel the same way whenever my boyfriend goes out as well. The thing is that I trust him 100% and I know he would never do anything to hurt me but I still feel a general sense of fear and anxiety whenever he goes out. Something that I find helps is to make plans with friends to go out when I know my boyfriend is going out too. By making plans with friends, it not only helps keep me distracted but also makes me feel better about the fact that I'm not out with my boyfriend. Maybe communicate with your girlfriend about letting you know ahead of time when she has plans. But I know there are occasions when last minute plans come up as well. Another thing that my boyfriend and I do that helps is to call each other at the end of every night, no matter how late it is. Something as simple as a 5-10 minute call to say that we got home safe and that we're thinking about them can be so reassuring. Just knowing that my boyfriend will be calling me when he gets home is something that helps me with my anxiety and fear.