A grand jury is just a bunch of people who are sworn in and look to see if there is enough evidence for a trial. I would think some of whether a grand jury would happen or not would be cost? If no one on your abusive boyfriend's side of the fence wants to charge you, the only one's who would/could would be the state/locality and, in a relatively clear-cut case like yours, who would want to? They'd first have to decide if it was worth it and then, if yes, they'd have a trial. But why bother deciding if it's worth it if no one cares?