Until they (doctors) have you sorted out, you are in an uncomfortable cycle of problems. The reason behind your heart murmur might make it a little more difficult to breathe during vigorous exercise, this makes you anxious and breathing becomes even more difficult, which makes you more anxious, and so on. Since breathing into a bag, and so decreasing the oxygen in your system, made you feel better, I doubt if your heart condition is bad. If it was, reducing your oxygen in order to reduce your stress would have made you pass out. As you have read, there are lots of us here who have to deal with anxiety disorder, and we've all managed to do so. I'm glad that your doctors understand that they have to treat both medical conditions at the same time.

When is your echocardiogram?