I do not think you have a chemical imbalance, per se. There are things you need to check here.
It is my consideration you are having anxiety attacks when you are around people. You have an addictive behavior, which means you have less than adequate self esteem. Addictive behaviors, such as smoking, etc., are rooted in a behavior meant to follow the crowd, make you feel empowered, and to change your system, i.e, smoking causes chemical changes in the body, and of course, drugs and alcohol inhibit or exacerbate your behavior, giving you a sense of belonging and a boldness you cannot experience on your own. This is what you need to work on to change, since you suggest you are not into drugs anymore - which I hope you are not. Please do not change one drug for another or get into any other addictive behaviors as this just masks your problem.
Therefore, your problems are emotional, unless you haven't had a physical lately and had such things as your thyroid checked. You might consider this first. The thyroid controls major internal parts of your body and your emotions, to include your control over your behavior, to a certain extent - this I can promise you.
Talk to your girlfriend and tell her what you are going through. If she really cares about you, she will understand. As an example, albeit we'd been married for over 10 years, I had a hyperactive thyroid and have screamed, yelled, thrown things and said things that I would never have said or done for quite a few years and my husband, my family and I have talked about it, with me explaining that I cannot control this behavior. My husband and family have been understanding (even when I threw my high heels at my husband, and once I threw 2 cans of vegetables at him at full volume) and they have supported me. This was a very serious disease I stuggled with for about 5 years and didn't know I had for longer than that. Of course, I don't do this anymore because it has been treated.
My suggestion to you is to work on this problem at lightening speed. Talk to everyone you can about it, your parents, your siblings, your girlfriend, your preacher. Find a group that you can talk to, such as group therapy or a help group within your work force. Employee Assistance Programs can help with this, find a social worker to talk to about this.
Another way that might help you is to learn meditation. Some people may think this is crazy, but it is not. It centers your mind and allows you to get your act together. Find some meditation tapes by, say, XXXXX XXXXX (he has a set of 2 out there that deal with the chakras and a multitude of meditation techniques that are very helpful). Rodney Yee has an excellent tape that I am sure my colleagues are tired of hearing about because I suggest it so much. . . it is called A.M. Yoga, takes 20 minutes and will center your day. It is extremely cheap and you can find it on Gaiam.com or just search for Rodney Yee.
Do not just go along with this behavior as it will follow you around until you face it. Deal with it now and get through it. One thing you might see as positive, you are highly sensitive and extremely care too much about what people think of you. There are a lot of us the same way out there. I do wish I had my husband's outlook on life, which is "hey man, I don't care if you like me or not, deal with me or get outta my way" and EVERYONE loves and respects him. I can't do that. The only thing I can do is what my Danish Great Grandmother has passed down through the generations - which is what I will tell you - "Pull up them bootstraps, man, and know that you are perfect just the way you are in God's eyes and deserve to be loved, liked and to have the self-esteem of a rock star!"
I hope this helps and stick with it. Good luck and God Bless.