Heart phobia

So for the past three months I have had an unexplained fast heart rate of 90 bpm, when my normal is 60 bpm. I have had ECGs, ultra sound, x-ray, blood test... everything comes back good (except that blood tests suggest hyperthyroidism but my doctor brushed it off) and now I fear that I'll have a heart attack or something since I have read stories about young people (i'm 18) having heart attacks suddenly and I don't know what to do in order to not fear it's really overwhelming me.

I turn on the TV and the channel is Disney. There I see happy teenagers chatting, enjoying life without any fear and I'm like... what have I done to not be like them? I have never smoked, taken alcohol, taken drugs, no previous health problems, nothing. Ehhh.