Hi,
I've never actually blacked out but I've ended up in the ER / A&E a few times due to weird heart behaviour, including being taken away in an ambulance straight from work feeling very lightheaded, so I can understand where you're coming from. I was 23 when it all started and am 26 now, so have had the same problem you're facing - some doctors assume there's nothing really wrong with you because you're just too young.
I am still here over three years later, so my advice to you would be:
1) Don't assume you've got a serious heart condition until you find out more about what's going on in there - dwelling on the worst-case scenario will just stress you out without accomplishing anything positive.
2) Be persistent in demanding appropriate testing - I'm not sure where you're at with that, but hopefully at least some investigations have been done at this stage?
Have you had a complete workup by a cardiologist and seen an EP?
I would ask to have a head up tilt table test if you haven't - to see if they can find the cause of it.
Look up Dysautonomia - or autonomic nervous system dysfunction. Postural orthostatic tachycardia syndrome (POTS), neurocardiogenic syncope, mitral valve prolapse dysautonomia, pure autonomic failure, autonomic instability and others are part of this disorder.
Here's the link for the forum here:
http://www.medhelp.org/forums/Dysautonomia-Autonomic-Dysfunction/show/266