I too have heart fears and can sympathize with where you are - I've had every heart test they can do without surgery - luckily all is well. While the body's circulatory system is amazingly complex, its also pretty simple. Your heart is little more than an electric pump which moves blood around your body and basically 3 things can go wrong.
1 - The electrical system which makes your heart run can be faulty (thats why some people get pacemakers)
2 - The heart muscle itself can have some form of defect (faulty valves, weak tissue, etc)
3 - There is a clog somewhere in the pipes (this would be your cholesterol)
The good news is that these things are actually pretty easy for docs to diagnose. The hard part is that we heart worriers don't believe they're easy to diagnose and are confident that they missed something. My doc told me, that once you're given the all clear, short of some form of sudden massive physical trauma, disease or other external source, its takes years for changes to occur which can cause problems.
So you've been checked out and deemed OK - thats great! Now make sure it stays that way by eating right, exercising and seeing your doc for your annual check-up and follow whatever directions he gives you.
Best of luck!!