I know that its not all cut and dried is it. We cant turn our feelings off no matter how they drive us nuts. Maybe you miss the security of knowing that he is 'there' for you? Clearly his gf doesnt want you in the background should you ever change your mind and say lets give it another go.
You seem to have been seperated a long while but have remained friends. Could it be the friendship that you miss? I think sometimes while we have an element of uncertainty we kind of get stuck because we are fearful of both moving forward or going back. Maybe its a normal reaction to the possible loss of something even if it is more the friendship than the chance of what could have been. Id be pretty devestated if I had to lose one of my friends. It would still hurt.
Maybe you could find a way to have one final talk with him just to clarify things between you?
Good luck.