Oh my, Amen. I know how you feel!
I am 17, and like 2 months ago, I was at my ex-boyfriend's house. I wrote something stupid to his friend on his Tumblr, and he tried to get the laptop off me, but when he got it, I let go and I flew back and I actually did smack my head off of a solid wooden bed-frame.
I was convinced I was dying by the 3 week mark after the accident because I was lightheaded EVERY day (I am right now as I write this!) and it began to worry me so bad that I had my ex drive me to A&E on a Sunday night! We sat for about an hour and a half and I got tired of waiting because I had school in the morning - so I got up and spoke with a doctor and he asked me some questions like was I knocked out or have I been vomiting/passing out? etc. And no, I hadn't. So he told me I could head home and come back in the morning - but to come back to hospital if I was going to be sick.
Now, knowing what anxiety is like... if I am told that the danger is in vomiting, then obviously my brain is going to make me worry to the point I think I am going to throw up. I ended up sleeping in my mum's bed crying from worry thinking I wasn't going to wake up in the morning... but I did!
My mum took me to A&E after school the next day. After 5 and a half hours, I was seen by a doctor who did some tests on me like checking my pupils were the same size, checking my ears, my pulse, blood pressure, co-ordination/co-operation speeds and my skull for any dents or cuts. I turned out to be fine, considering I hit my head VERY hard, at the back too - very sensitive part of the head that.
But yeah, it's now been 2 months since the accident and I STILL feel lightheaded. It's a pain in the neck (or should I say head, lol) but it worries me because I get sore where I hit my head and I get REALLY sore headaches sometimes. I also thought I had a brain hemorrhage but luckily (thank God) didn't.
The same thing happened in 2012 when I over-dosed and had a 4 hour severe palpitation and low blood pressure attack - I was convinced my heart was screwed after that and it'd palpitate when I wasn't even thinking about it. But now it isn't even in my sub-concious anymore and it only palpitates if I drink too much coffee
You definitely aren't alone. I always panic about getting cancer or some other dangerous disease, cuz I am petrified of dying young. Like so scared it makes me sick!! Ok shutting up now but at least you know you aren't alone