What Causes Breathing Difficulty Along With Dizziness?

Feel oxygen deprived from just speaking to my high school students, then my voice fails and squeaks out, then I push from the low diaphragm just to finish a sentence in a near whisper/gasp. I have to pause, forcibly take a very deep, intense, saturating breath to regain my oxygen, then another....so I can not pass out from lost oxygen light headedness. I have diagnosed valvular heart disease based on workload of 3-5 METs resulting in dyspnea, fatiague, angina, dizziness or syncope, and left ventricle dysfunction with an ejection fraction of more than 50%. When diagnosed per above, I had NO feelings of oxygen deprivation, lost voice or lost breath... but now, three months later it is daily and constant. Help?