Hi Mcb5196.

To me, it sounds like you're either the dismissive-avoidant attachment type or there is a feeling underlying the nagging one.
If you think that your thoughts are dictating the way you feel, which is usually the case, I'd take time to listen to these thoughts in the position of an observer/listener rather than the receiver. It could lead you to discover what is the actual feeling you're experiencing. Because you're mentioning the feeling tend to fade while being with your girlfriend and so your thoughts aren't overwhelming you, it indicates to me that it may be your thoughts.
You could also seek a therapist in order to find out where this anxiety comes from. Trauma? Childhood events?