Just a phase? or something more?

i am a 14 year old male and get plenty of exercise. I dont always feel like this. It's the worst when I think about it, thats when it feels like im just going through the motions, like im in a drema or something. I know im doing things but it just doesnt feel like it. like i know im typing this but it just doesnt feel like it..if that makes sense. I think im like going crazy, when i think about this i always think im going to die for some reason of some undiagnosed condition. like if my chest hurts i think im having a heart attack or gonna go into cardiac arrest. anyone knows what this is? its the worst in the morning and at night or whenever i think about it, at others times i feel fine and live a normal life but when im thinking about this i feel awful and think im definitely gonna die any second..could i be a hypochondriac? anxiety? depersonalization? i went to the doctor a few times..they said its probably anxiety and i went to the cardiologist and they said my heart is fine..someone please help i cant live like this OH and one more thing when something feels wrong i always have to google symptoms of something to see if i have them..this has been going on for about 4 months