Chest Pain, Shortness Of Breath, Dizziness, Palpitations, On And Off Blurred Vision. Detected Sinus Tachycardia. Fatigue After Taking Diltiazem. Help

My husband is 25 years old. He has been having chest pains for years now. He also has shortness of breath, palpitations, dizziness and blurred vision off and on. Dr. after Dr has not really concerned themselves with his problems. Telling h it is in his head. He is in the Army and can't even do his PT, or regular workouts without having to stop before he passes out. The cardiologist he sees now is really rushed and doesn't explain things very well. Well, he put him on a 30 day moniter and was diagnosed with sinus tachycardia. They prescribed him diltiazem. The only thing that medicine is doing is making him extremely tired. It isn't working for him. Since starting this medicine, his symptoms have only gotten worse. He can't even walk to the end of the driveway without feeling like he is going to pass out. How can we get a doctor to finally listen? I am so worried about him. Can he die from this? Would his high cholesterol have caused this? What other medicines are there that could help?