Why does the combination of having ADHD and anxiety make one feel that rest is unsafe?

Even on weekends, my body is constantly scanning for the next task. Rest feels like the same thing as a long-distance relationship. Perhaps ADHD makes a person to believe that stillness is synonymous with failure and that movement is life. The Anxiety then comes in and converts stillness into a threat. It’s really bad: the more I make an effort to relax, the more I feel that something is wrong. Has anyone else experienced that "calm" feels so strange, almost like a threat?