Although it may sound selfish, I have a really easy time of letting people go who consistently cause me pain - no matter who they are. Family, friends, lovers, children - whoever. My theory is that, if you're causing me pain, even if I love you with all my heart, then I have to get away from you because I'm no longer into martyrdom. Did that with my first wife and got me nowhere except screwed up emotionally, financially, and spiritually for several years - and that relationship was only 2 years long!
I admire people who can hang in there but I know that, for myself, after the pain and abuse reach a tipping point then I am gone - not interested in healing or making amends or building bridges. In fact, I usually nuke the bridge as I'm leaving just to make sure I don't go back over it.