How can I control my repetitive negative thinking when I'm feeling anxious?

You can do this by simply living in the present and not worrying about the future or the past. Negative thinking can lead to destructive behaviour so you can break the habit of this by simply trying to focus on something positive anytime you think negatively this will then break the cycle. A way to do this is by simply closing your eyes and focusing on your breathing. Notice each breath you take and how you feel in the present moment. Ground your feet and rest your hands on your lap and just take a moment to notice all your features starting from your head down.