How does one handle an ex-wife who refuses to move on?

My fiancee's ex divorced him four years ago. I met him after their divorce. She hasn't moved on and refuses to let their son (16 years old) meet me, visit with his father and me, or have any contact with me. She is still bitter and angry over their divorce, even though she was the one who divorced my fiancee. My fiancee tries repeatedly to set up visitation with his son, yet he has not had his son for a weekend (as mandated in the divorce decree) in over two years. Plans are made and at the last minute, the son or his mother cancels them. My fiancee calls or texts his son every day and tells him how much he loves him, but it's often weeks before his son responds. It's repeatedly suggested that my fiancee shouldn't be "out dancing," "out dining," or "living with" me and that he should be buying his son's schoolbooks, clothing, etc. (My fiancee has child support deducted from his paycheck and in the past has had MY money extorted by the ex--hundreds of dollars--for things that the son purportedly needs, over and above child support, when my fiancee is bankrupt, can't even afford to file for bankruptcy, and works at a minimum-wage job, and I'm disabled and work only part-time. He walked away from the marriage without even getting an attorney and gave his ex everything--even the house that he worked for for 15 years. It's emotional blackmail, using the son as ammunition against my fiancee.) His son, I believe, has been brainwashed against me by his mother. My fiancee baked a birthday cake for his son and dropped it off the day before the birthday, but wasn't allowed to go and participate in the birthday party. He never even got a thank-you call from his son. We're not invited to any family events, and at the one that we were invited to (a christening), we were rudely ignored by the ex-wife and son. Doesn't this woman realize that I'm going to be in the picture for the rest of her son's life? Doesn't she realize that by making her ex-husband's life a living hell, she's damaging her son's relationship with his father and damaging her son? Doesn't she realize that I didn't ruin her marriage? What am I to do in this situation? How does one handle a woman/situation such as this? Is it possible that some people hold onto anger forever? Why am I the brunt of it when I walked into this situation after their divorce?