from what you've described, this is a normal issue for a couple to have even when both are mentally healthy. the trick is in finding the balance between spending time with your girlfriend and spending time with your friends.
sometimes it can help to establish a loose schedule, such as, friday night you have a standing date with her, and sunday you always hang with your friends. the rest of the time can be flexible.
it's important to make time for your girlfriend, but she should also understand that you value your friendships as well.