boundaries... just can't seem to get it

Boy I can sure pick em can't I?  My therapist says I have some interesting 'characters' in my life.  And that I have got to continue to work on boundaries with people I meet.  She says that for some reason I did not develop this skill as a child b/c of the abuse I experienced as a child.  I asked her if there was something I could read to help me b/c i just don't seem to get it.  I like people who are kind to me but apparently not everyone is on the up and up. And if I have decided that someone is my friend I am loyal to them regardless of whether or not they are good for me, hurting me, or sometimes I have just had enough.  But of course I always feel to blame b/c of the end of the 'friendship' whether it was my fault or not.    She said there is no book and that  I have to develop it on my own and listen to that little voice... But  I just seem to have this sign on my forehead that says 'take advantage of me - use  me - abuse me & throw me away when you done with me'!  I gotta get some new friends...