My boyfriend with someone else makes me sick...?

University Students first of all. We are going through a rough patch and I'm afraid he's going to break up with me and immediately hook up with this perfect perky girl in his apartment building right after we do break up. The thought of this makes me physically sick, vomiting and all. Whether we do break up or not is a separate issue, but if we do break up, how can I not be devastated by the thought of him with her, especially if I see them together on campus???