From what I just read, SVT can be caused by high stress levels.
Now, you've had all of these tests that say your heart is normal. Why the heart worry? Do you have a family history of heart issues? Going by your picture - you look young! Heart worries are for later in life you'll have plenty of time for those way down the road!
We all have our own specific fears though, some are afraid if heights, some are afraid of neurological disease (that's my fear), and others heart problems.
Based on the numerous tests you've had, I'd wager you're just fine.