[cacophony]

while it certainly is possible that you're showing signs of asperger's type qualities, it may be a lot less clinical than that. you referenced that you've lost the ability to trust, and you followed up on that statement by mentioning that you have no problem socializing with coworkers because that becomes a "responsibility" or "work" thing.

it sounds like you've come to associate social interaction with vulnerability. there's something in you that you don't want anyone else in the world to touch. hell, you may not want anyone else in the world to see it, much less touch it. you've built a wall around yourself to keep this thing safe from the outside world, safe from exposure. the easiest way to prevent exposure is to restrict your interaction with people so that it never progresses beyond the superficial. as soon as the superficiality is breached, you run the risk that your wall will be breached. thus, a quick recoil and a serious disinterest in furthering the social interaction. your sense that people lose interest in you after the initial "getting to know you" chit chat may be a projection of your own feelings. or perhaps you're a bit more overt with your growing discomfort than you realize, and your conversation partners choose to disengage when they pick up your vibes.

ultimately the only way to get to the bottom of it is to try to figure out what it is that you're protecting from the outside world. which is easier said than done. you mentioned that you want to avoid counseling, but frequently having someone to talk with about your feelings is the best way to get to the bottom of your issues. and your girlfriend isn't enough. she's there to love you and support you, not be your therapist. it might be beneficial to your relationship if you do decide to seek some counseling because your girlfriend won't have to be the one bearing the burden of your soul-searching.

just my 2 cents. armchair analysis.