First, I've never been in this situation, so I can't imagine how you feel. Like many girls, I've come close to having it happen, but thankfully I was lucky and it didn't. (Can I say thank God for good guy friends?)
And I'm sorry you have had to suffer through such an experience. But you seem to be handling it in a very mature way, and you are making the first step by realizing you can't get over it on your own. The next is to find some support for it. It doesn't matter if it is a good friend, a parent, anyone that can support you while you try to get past it. Then, you find a therapist, or a program to help you. Therapy is needed so that you can work through the issues it has presented in a healthy way. And while I, like many people, have an aversion to therapists, they can really help you in a situation like this.
You also have to get away from those who are giving it a negative connotation. Your step-dad, for one. ANYONE who thinks that rape is the woman's fault is completely ignorant about the act of rape and doesn't understand it at all. He won't be able to be a supportive, understanding influence while you work through this and you'll have to get away from that if you want to try to heal.
But working through it is only one of many steps you are going to have to take. No matter the therapy or the support, this will be with you for the rest of your life, and it will affect it. Once you work through it, and decide you are ready to date and try to be a sexual individual again, you'll have to find a man that is caring, understanding, and willing to work to be with you. You'll need a healthy relationship with a guy that can truly be there for you. But that is a long way off.