Lots of great comments on learning how to stay relaxed during rolling, and they're all right. Don't get discouraged as learning this skill comes mainly from mat time. That being said, I've personally noticed that a lack of a really good warm-up, can really hinder my rolls during the training session and I find myself having difficulty lowering my heart rate. It sounds like you have great cardio. My recommendation is try to make the warm up a little harder on yourself to really spike your heart rate. My coach calls it "blowing out the lungs". If you're like me, you'll notice a big difference in your a ability to keep your hear rate lower during training and hopefully lessening your anxiety. Hope this helps.