Feeling like I'm dying and no one believes me

I think I might be actually dying. The last 7 months of my life have been absolute torture. I’ve been to a cardiologist, the ER countless times, my family doctor, and everyone wants to say I’m fine. That it’s in my head. Last night I was laying down and my heart all of a sudden started racing and it took forever to get it to slow down. I woke up feeling okay, but as I was going for my morning walk same thing. It doesn’t make sense to me that this is panic or anxiety when I’m doing regular day to day tasks. Dying is one of my greatest fears and I do not know what else to do.