You two have been together since you were only 15. Apparently you weren’t “dating” when you started to have sex. At that time, sex was coupled with the excitement of the new and the forbidden. You don’t seem to think that anything has changed but something crucial has. The two of you are now entering adulthood and life is much more complicated. Your girlfriend may be taking a step back and re-deciding if you’re the guy for her. She may be thinking that too much of your relationship is based on what happens in the bedroom. She may be wondering why, at only 23, her life has no changes in it.
It’s time for a talk, not about sex but about where you each think you’re going with the relationship and with your individual lives. Are you ready to make a lifetime commitment? If not, why not? Are you each satisfied with the way your futures are unfolding? Do you share goals and dreams? A disconnect around these issues may be making it hard for your girlfriend to feel close to you.
If, on the other hand, the answer is yes to all of the above, then it may be time for your girlfriend to see a doctor. Medical conditions that can cause a drop in libido include hormonal problems, onset of diabetes, and consequences of alcohol and drug abuse, for example. Depression can also be a factor.
I hope the two of you can take stock of your lives and your relationship with openness and caring.
I wish you well.