How to deal with the disappointment I feel toward my mother?

My mother has been in an on-and-off relationship with someone. At first, he seemed like a good guy, except for the fact that he didn't appear responsible when it came to driving, as he had been in jail for driving under the influence. However, he seemed responsible in his work. It wasn't until later that I discovered the real reason I hadn’t seen him for several days or even a week: he was actually staying at another woman's house. My mother and I found this out when the woman came to our place, believing he lived alone.

At one point, she tried to break up with him because he always went back to this woman. However, she hesitated because she thought he would finally decide to stay with her. For a while, he did, until the other woman came looking for him at our place. Legally, he was not allowed to come close to her because he had hit her during a fight. Despite this, he went to talk to her, which resulted in an argument between my mother and him.

When he decided to return to the other woman, my mother finally made the choice to leave his belongings at her house. Even after I advised her to block him, she remained in contact with him. Last night, he knocked on our door, and my mother told me to open it for him. I refused because I was frustrated with her; it's been years of this cycle, and she continues to lower herself to someone who doesn't seem to respect or care about her feelings.

Now, I am disappointed in her for not showing more self-respect and for not standing up for herself by preventing him from entering our lives again—especially knowing that he continues to send money to that woman instead of fulfilling his responsibility by paying child support to his ex, who is in another country.