Hi Zelda and welcome to Forums!
You are most certainly not alone. Most people here will understand what you're going through. We all face different phobias and try our best to deal with them.
My advice would be to of course go and get the tests done. So you can prepare to stay strong and beat whatever you might have. Get to know your 'enemy', what you're up against. I'm referring to your heart condition there. Other fears are at this points catastrophizing thoughts, flooding your mind.
You will get through it and you will be so proud of yourself later. Good luck and keep us posted