You have been conditioned by society to view the possibility of a gf being with someone else as a negative, "bad" thing. Many of us have been taught that if someone we care for enjoys the company of another, it must mean we are inadequate or lacking in some way. These are just "tapes" playing in your head as a conditioned response to certain situations or triggers. We all have thoughts that come up in various situations only because we've been conditioned to think that way - and it is the thoughts that brings up certain feelings as a response.
Basically, you are starting at a good place because you are acknowledging these thoughts. Now your task is to see them but do not entertain them. Pay them no credence.
Marcus gave good advice on questioning the logic in your thought process. It may be that those thoughts never go away because they are so ingrained in us, but they do not have to be so loud, terrifying, and crippling. Eventually, by continuing to look at this stuff and getting to know your own thought process and logic system, you can recognize them for what they are and have some distance, as in, "Oh, there are those thoughts again telling me she shouldn't want to be with anyone." And then you occupy yourself with other things, something constructive, until those thoughts are just like a pattern of wallpaper, in the background and not getting in your way.
Knowledge is power, and self-knowledge can give us so much!