From an outsider looking in, you appear to be codependent. Have you read up on codependency before?
Just curious what your childhood was like. Are you parents still together? Were you ever abandoned as a child?
You continually ask why you have such a strong pull towards someone who has repeatedly cheated on and left you, I think exploring your childhood might hold some clues for you.
Also, look into attachment styles. Clearly you have an anxious attachment style, that might be something to look into as well.
