Thank you for contacting JustAnswer.
I am sorry to hear about the problems your daughter is experiencing. It is extremely difficult to be in an abusive relationship, particularly because things are never all bad or all good. You are correct that abuse is often about control. Your daughter's boyfriend may at times act in ways that are not abusive, making it difficult for her to tell when he may revert to abusive behavior. You are also correct that you cannot make her decisions for her, as much as you might want to tell her to get far, far away from this man. What you can give her though is unconditional love and support. You can also encourage her to go to individual therapy as well in order to get some professional help in making sense of her choices, build up her self esteem, help her to set boundaries, and perhaps address any patterns of behavior that may be occurring across your relationships. She might also benefit from Alanon meetings that are for family and friends of those with addiction problems. You can certainly remind her that no one deserves to be abused. You both may find the following website helpful: http://www.mentalhelp.net/poc/center_index.php?id=2
Remember that she will have to make her own decisions about the relationship. Unless you believe that she is in imminent danger, you can simply be there for her. In the meantime, be sure to take care of yourself through healthy eating, exercise, rest and reaching out to supportive people in your life. I hope this is helpful.