Can't take a full breath in, lungs wont stretch?

Okay when i was born i had 2 holes in my heart, but they closed up before i was 2, but i have always had a murmur in my chest, im 15 now, and i cant take a deep breath in at all, you know that sensation when you take one in? i dont get that anymore, just uncomfort and the feeling of lacking oxygen in my lungs, when i went to the doctor the last time i didnt mention anything but because of my murmur they did an EKG hich was really fast, and performed a heart rate whilst sitting and thens tanding up, my heart rate dropped dangerously low when standing, it honestly feels like i can only fill my lungs up 3/4 of the way up, i cant yawn, or anything, its getting so bad i feel dizzy and absolutely exhausted, please help me, NO its not anxiety, its so much different