How can anxiety cause such severe shortness of breath? how can the chest discomfort and breathing difficulty be so bad if i have good heart/lungs?

Wonder if you are: possibly having panic attacks? Anxiety can lead to constriction of smooth muscle in airways which makes breathing difficult. You may also be hyperventilating.

"panic attacks": Extreme stress, whether from acute fear or internal turmoil such as anxiety, can trigger the release of stress hormones. The immediate hormone is adrenaline which prepares our bodies for "flight or flight." Heart rate and respiratory rate increase, blood flow is diverted from internal organs to the skeletal muscles When there is nothing to fight or flee from, the symptoms you describe occur.