Hello, I'd like to help you with your question.
It sounds like you have been through a lot. Having so many significant people in your life leave is a devastating loss. And the effect on you would be significant. You have not been able to bond with anyone or have anyone provide positive reinforcement to you, except in your current relationship. That can cause a lot of deep pain.
One thing I noticed is that you take a lot of the blame onto yourself for everyone leaving. But I get the sense that you have been exposed to a lot of people who have their own issues and who are unable to connect with others. Your father leaving probably had a lot to do with your mother or other circumstances rather than you. Children cannot cause a parent to leave. And mothers usually don't just up and go unless they have issues of their own. You didn't do anything to cause her to go.
But because your parents left and you had to live with an unstable step mother who hurt you, you had no choice but to take all this in and assume that you were the cause. As a result, you may have developed a fear of being hurt again and low self esteem. That can cause you to react to others with fear and defensiveness. Most people will not understand what you went through so they will assume it is about you and not your pain and in turn react in their own way, possibly being mean to you.
You mentioned that you have a long term relationship with your boyfriend. He understands you and accepts you for who you are. This is proof that it is not you that causes others to leave. You have just been surrounded by others who may care more about themselves than anyone else.
Because of the deep pain you have experienced and the hurt and low self esteem you carry with you, you may want to consider talking to a therapist to help you sort out your feelings and find ways to cope with all the losses you have suffered. To find someone to talk with, ask your doctor for a referral. Or you can search on line at http://therapists.psychologytoday.com/rms/.
You may also want to work on your feelings by learning more about loss and the pain you feel. Here are some resources to help:http://www.helpguide.org/mental/grief_loss.htm
Self-Esteem: A Proven Program of Cognitive Techniques for Assessing, Improving, and Maintaining Your Self-Esteem by Matthew McKay and Patrick Fanning
The Six Pillars of Self-Esteem: The Definitive Work on Self-Esteem by the Leading Pioneer in the Field by Nathaniel Brandenhttp://psychcentral.com/lib/2011/self-esteem-struggles-and-strategies-that-can-help/
I hope this has helped you,