Anxiety help plz

The past week, I've had horrible anxiety, about everything. Like I'm always thinking way too much and assuming everything had happened for the worse. Example-my boyfriend says a phrase that I've never heard him say before. The first thing I think is, "Hes hanging around new people, so it had to be a girl, so Hes cheating on me with her. Why would he do that to me? When I know its not the case. Thats just an example, I get really anxious about school, and coming home, and sleeping. What do I do? I need advice.