I have no medial background whatsoever, but my advice would be to seek medical help asap. But i know how hard it is to afford for most of us, so maybe that is not an option; I don't know your situation.
Is there anyone who lives with you that you can talk to this about? If not, you might want to inform a neighbor or something.. or at least tell someone or everyone close to you so they understand what might be going on if you need serious help in an emergency.
Do you try breathing/relaxing techniques at all? My best recommendation would be to go to the library and spend an hour browsing through books that might help you. I have found the library as a valuable resource when dealing with all types of life issues. Maybe you should look for one about mental relaxation.. so when your body is physically numb you are able to at least control parts of your mind that might be conected to what is going on with your body

What do you mean when you say that your heart stopped?
Do you mean that you could not feel it?


The condition of not being able to move in your sleep that you are describing is called Sleep Paralysis (Try searching the site to compare your experience to how other people describe it).

It is usually caused by stress and can be cured with relaxation and breathing techniques.

Here is a helpful site describing the condition.