I'm a lot like you, Nick, and maybe even a little worse. I've struggled with irrational, soul crushing jealousy and still struggle with it currently, but it's not as bad as it has been before.
Like Arch mentioned, you should probably take a good time out and think about what exactly is causing you to be jealous. I found for me that it roots in my own personal insecurities and low self esteem. I think you jokingly mentioned something along the lines of why would your girl pick you, which is something that personally fuels my jealousy. I can't really figure out why my girlfriend is with me and so content with me, so I figure that's where trust comes in.
I've learned to trust that she's with me for a reason and doesn't care about other guys, I may see myself as less attractive or less whatever than whoever else, but she doesn't see that, she just sees the person she loves and the person she chose to be with.
Also I think someone mentioned telling your girlfriend about it, or asking if she knows about it. You should definitely mention it to her if you haven't already, and tell her how much it upsets you and ask her to work with you on it. Like I mentioned above, my jealousy has toned down a little and it's mostly because my girlfriend has shown me that she's not interested in anyone else, and it wasn't anything major she's done, it's her overall dedication. You should focus on little things your girlfriend does for you, and not the little things other people might say to her.