Hi,
Your potassium levels should be monitored properly and treated. Without being a doctor, I think that many of your symptoms can be explained by this.
Electrolytes (Calcium, sodium, potassium, and to some extent, magnesium) is important to make your cells (incl. heart) work properly (at all). Hypokalemia (too low potassium) can potentially trigger serious heart arrhythmias.
I don't know how low your potassium was, but it's very important that you get this treated, and if possible, find out what the cause for low potassium is.
You've been dx with a micro valve prolapse. Is it a micro (very small) valve prolapse, or a mitral valve prolapse? I doubt this can explain your symptoms, but it's important to find out.
I think your problems can be caused by some anxiety as well. Anyway - first, and most important, your doctor should find out why your potassium is so low - as this is a possible dangerous condition.
If there were something wrong with you they wouldn't of let you home, its more than likely anxiety, i have it, i get breathless and feel like someones squashing my chest, i get palpatations, ectopic beats it goes on......its a horrible thing to have,ive been a and e, i think you do what i do you think about it too much, wonder if the docs missed something, try not pay any attention to it ignore the heartbeats and just breath normal, thtats what i did it worked for me but takes time BUT as soon as you start about your heart again anxiety comes back, trust doctors they know what they're doing,