" She tells me she loves me and if I don’t say it back she will make a big deal out of it." -- I am a mother of two adult children--when I tell my children I love them, I never expect them to say they love me back. I am spontaneously expressing how I feel at the moment. I feel like the best kind of love is very accepting and not very controlling. You said you are respectful, you do owe her that but it doesn't help anyone to fake what you really feel.
"Should I be genuine and try to distance myself from her emotionally?" We can never totally distance ourselves from our mothers even if we want to. That is just nature's programming. But I do hope you don't live at home given your mom's abusive situation. It's her choice who she wants to be with. You say she knows how you feel. There is nothing more that you can do. I hope you don't live at home so you can distance yourself by keeping in touch through an occassional phone call, text message or email. If she asks you to visit, tell her that you don't want to be around her boyfriend but encourage her to visit you instead. If you live at home, I am SO sorry, this sounds terrible (can you get away from there?). If you don't, an occasional phone call will let her know you care while allowing you to stay away from this dirtbag. Please know that this situation is not your problem or your responsibility. Never feel guilty for other people's choices. We can only be responsible for our own actions, not others, no matter what guilt trip people try to lay on us! I think that sometimes people get depressed because they take on the guilt that others lay upon them or worry about things they cannot control. Your mom's situation is not your fault. Always remember that!!