You aren't chemically imbalanced, you have been emotionally damaged and your sexual behavior is an aftershock of the damage you suffered. The only way you are going to heal your problem is to stop acting out by having one night stands. When you make the decision to stop acting out, the true issues and problems will come to light so that you can begin to heal.
Let me explain. When a person engages in any compulsive action (in your case, one night stands) the mind is going into action in order to discharge painful feelings. While the action may temporarily relieve the pain inside, it creates more suffering down the road. Your first step is to stop acting and start healing the feelings that come up when you give up this pattern of sexual acting out. In order to stop the sexual acting out, you need to ask yourself, 'What feeling am I afraid would overpower me if I stopped these one night stands?' Am I afraid that men wouldn't like me if I didn't give them sex? Am I afraid of feeling empty, unwanted, unloved, alone, or what?'
When you figure out what feelings you are running from, then trace the origin of these feelings back to your childhood. Surely you felt unloved as a kid. If you felt starved of love as a child, you probably figured that there was something wrong with you (that you were defective and unloveable). When a girl feels unloveable, she falls into the kind of behavior that you are engaging in now. She doesn't feel worthy of love for who she is, so she gives her body away, hoping that her gift will win her love. Her gift is met with disdain, since a man doesn't respect a woman who doesn't respect herself; she gets dropped, which hammers her self-esteem even lower.
Not knowing how to heal herself, the woman falls into the only pattern she knows, she sleeps with another man, hoping that this time she will finally find love through sex. This is a major vicious cycle that has failure written all over it. In order to heal the pattern, you are going to need to have the courage to stop acting out and to tolerate the feelings that come up. You are going to need to find out where this pattern began and what purpose it is designed to fulfill. You are going to need to find out who made you feel unwanted as a child, and whose love are you trying to win through these one night stands. You are going to need to learn that you are deserving of love and entitled to ask for what you want and to be responded to.
Most of these lessons are going to have to be learned in a therapeutic relationship. In that relationship, you will feel loved the way you didn't as a child, encouraged to say your thoughts, feelings and needs, and experience the joy of being responded to. Your self-esteem will rise, and you will learn that you deserve to be loved and respected. Finally, that relationship will be your model for the relationship that you will form in your outside life.
If you can't afford to do individual therapy, then you can form healing relationships in group therapy. Until you heal a bit, try to avoid any more encounters with men. These encounters are only damaging you more and more.