I had a very crappy childhood too with a mom that really made me feel like a giant burden on her. I felt like I had no voice growing up for fear of my mothers wrath. I did everything I could to stay under her radar and keep the peace. As an adult, I was never able to really assert my own feelings or boundaries or felt comfortable asking for really anything. Any attention I received I clung to and wasn't even quite sure what to do with. I believe what happens is that as children growing up in these dysfunctional situations we develop ways to adapt/cope into our surroundings and this way of life becomes so very ingrained into our thought and behavior processes that it is as second nature and involuntary to us as breathing is.
As children we are taught where we fit in in this life and how to act. Breaking away from those patterns, no matter how unhealthy, takes a LOT of internal work and self awareness.