Help with 4 year-old abused dog

Hello! I have had my girl, Bella, for 4 years now. I got her when she was 4 months old.
While she has come so far from the timid, shy dog that would not let anyone touch her; she still does not trust me or anyone fully. She still cowers when approached, or if one of the other dogs are being disciplined, or when I offer her food...etc, etc.
She constantly(and I am not exaggerating the constantly) wants attention. If one of my other dogs is getting attention, she pushes her way into the mix. She will paw me (which hurts badly) if she is not getting attention.
I do not hit, scream, kick, or in any other way abuse or neglect any of them.
I do not know what else to do to help her become more confident or to get her to understand that she is safe with me. She does listen to commands, gets along with my other dogs, not strange dogs though. She does not take to strangers so she has not been socialized alot. She can be unpredictable at times. There have been 3 people, besides me, that she has taken to almost immediately in the 4 years I have had her.
Any help would be appreciated.