I am looking at this from the perspective of a mature woman, and a mom with adult sons, one of whom happens to be named Andrew.
First of all, your mom needs to stop looking for "a man to take care of her." And second, she needs to run her own life. I would be appalled if one of my children got into the workings of my relationships with any man, as you have. She is a grown woman and should not be looking for someone protect her and intervene in relationships when things go wrong.
With three failed marriages and this relationship down the drain, I sincerely think your mom needs some counseling. Just like women who are abused keep getting into abusive relationships, she is attracted to, and sending out signals to, the "wrong kind" of men. Again, I am speaking from experience here. After a particularly bad breakup, I decided there were a lot of common threads in the relationships I had since I was divorced. (And I was married for over 20 years to my ex-husband. I consistantly chose men with the same characteristics, including my ex-husband. Now I look at things differently, believe me.) If your mom continues to make the same mistakes over and over, you can expect her to "get hurt" by men who use her, over and over. While it sounds like the men she married had their own issues, she is also the one that chose to be in those relationships and to stick around and try to "work things out". Most of what has happened to her is indeed, her own fault if she chooses to let it continue.
Your mom should not be putting this responsibility on you, either. She sounds like she missed some maturity lessons along the way. It is NOT your responsibility to see that your mom is in the dating scene or has friends or even if she is happy or not! She should be the one you come to with problems or to seek advice and not the other way around. But, to put it bluntly, you need to butt out of this part of her life, no matter how close you are. Seriously, I hope you mom (and you, too) seek some counseling or you will wind up neglecting your own life and relationships in order to "take care of your mom".