How do I muster up the courage to report a parent who may be abusive?

I'd say to imagine your own children being subjected to abuse, and think about what you'd want them to do. Surely you'd want them to come forward. Safety is more important than maintaining relationships with toxic people. By reporting possible abuse, you have the chance to potentially spare people- young and old- a lot of pain and grief. Consider that someone somewhere has to take action, and if you do it, you can rest assured that you've helped make a positive difference in the world by intervening for someone who might be subject to abuse.

If you are a mandated reporter such as a teacher, then you are required to do it, so it can take the pressure off. You could also talk to a mandated reporter and ask if they could help out. If you are not comfortable because you think that you will be identified, you could talk to social services about your concern before you begin to talk about it. Another idea is to role play--without giving any identifying information--with someone else you reporting it. The more you do it and get more comfortable, it may be easier. Please do not be hard on yourself. These things are very difficult.