How do you forgive when the behavior is ongoing?

I know forgiveness is important in order to keep moving forward in life without harboring resentment that will hold me back. Because of this I try really hard to forgive, at least partly because I don’t want to be torn apart by my own ill feelings. Now I find myself in a situation with an individual who continues to lash out at me without end in sight and at every opportunity. I know I have to let his behaviors bounce off me but because of the relentless nature of his behavior I find I am having a difficult time doing so. So how can a person find a way to forgive when the egregious behavior is ongoing?