[PSY]

I think it's natural to feel nervous when addressing a difficult subject. There isn't much you can do about your feelings. What you could do, though, is think of a good way to start the conversation, then practicing saying one or two sentences out loud. For example, you could start with something like this:

"[Name of girlfriend], there's something important I'd like to talk about with you. We've already talked about it a bit, but I still have some questions and would like to get a better sense of what's going on and how you're feeling about it."

Then you could stick to simple yes/no questions:

"Did [Ex-boyfriend] force you to [action]?"
"Are you afraid of [Ex-boyfriend]?"
"Are you willing to come with me to the police station and file a restraining order so he can't hurt you anymore?
"Please be honest: do you want to sleep with him?"

Then some open-ended questions:
"Why aren't you willing to tell the police about what happened?"
"What are you afraid might happen if you tell the police what happened?"
"Why do you feel the need to be around [Ex-boyfriend]?"

It won't be an easy conversation to have, but I imagine not knowing where you stand with your girlfriend will cause more stress (and possibly lead to paranoia/jealousy) in the long run.