Those Mental Tapes we play in our mind

Why is it that the more negative messages we receive the more we hold on to them? I got a lot of bad messages growing up as a kid. From my dad who was never part of my life I got the message that I wasn't worth his time, and in turn I felt that I wasn't worth anyone's time. After all, your dad is supposed to be your biggest fan and when he isn't it must be your fault right? As a kid, I put that all on me, I didn't or couldn't put it on him. I know now that it wasn't about me, he was just a jerk. My step dad, who had issues of his own was extremely verbally, emotionally, and at times physically abusive. Growing up I was told on mostly a daily basis that I was worthless, useless, no account and wouldn't amount to anything, etc etc etc. Now I know that he was messed up, but he messed me up! Big Time. How do you undue that stuff. I know in my head that I am a good person, worthy of good things and that those things aren't true, but in my heart I am not so sure. I do self talk, positive reaffirmation, and try to dwell on good things, but I am afraid that those deep seated beliefs are too elusive.