I watch lots of horror films/read lots of horror stories but I'm never really scared (although I did once dream of my stabbing myself, it was really weird). I suggest you watch some different genres for a while and try to forget the horror movie by replacing it with something else. Or you could just avoid horror movies and not watch any at all.
You still have many years before dying. Just be careful of what the future might hold for you. Relax! Maybe stop watching horror movies, and watch funny ones. Do something exciting like go to worlds of fun or something like that.
I can't watch any horror shows or read any horror books. It disturbs me for days, and I struggle to sleep. Everything makes me paranoid and it just feels that something disturbing has managed to crawl under my skin, so to speak. So, because I don't like this feeling, I decided to stop watching horrors. I would merely suggest that you do the same. If it causes you horrible nightmares it's best to rather not watch it.
I also had disturbing nightmares once, and what helped was to surround myself with pleasant things. When I would wake up after a nightmare, I would still feel disturbed (I had really wacked-out disturbing nightmares) - what I did was put on some gospel music. The gospel music had a loving, cheerful vibe to them and that would ease my heart a bit. I would then just go back to sleep and sleep really well and peacefully after that. After a while my nightmares started to disappear merely because I filled my mind with more positive thoughts than negative ones.
It's a simple thing really: You have filled your mind with negative things (horror movies), so to fix it you have to fill your mind with positive things. The Bible tell us to only think about positive things - "brothers, whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things." Phil 4:8 - I've applied this and it has helped me quite a lot. Please try it. I had night-terrors and disturbing nightmares once - God helped me with them through this simple principle. Stop watching things that disturb you and start surrounding yourself with beautiful and positive things
It's normal to think of death. Apparently it's quite normal to think suicidal thoughts or have nightmares about it. Just there is a line between thinking and trying of course. I'm talking about in reality here.
Every one gets nightmares.
Also avoid watching them horror shows. That would help and give the mind some rest.
I really don't think thinking of your own human mortality is something that out of the ordinary, and many people have thought of what it would be like to die. If you do need mental help it's probably not for that but to try to get you over whatever special snowflake syndrome you've got going on there.
Perhaps you should stop watching horror shows then? If you have thoughts of dying only when you watch these shows...well, isn't a simple solution to just stop watching them? But if you have constant thoughts about dying (for example, when you're just drawing, playing a game, talking to friends, eating, etc) then I would seek help, for sure.
It's like me with horror films. I get very paranoid if I watch them. So to avoid this, well, I just don't watch horror films at all.
I'm not sure if this is the case with you, but like I said, you should start off by choosing what you watch.
If you're really concerned, why not talk to a guidance counsellor at school?
Avoid horrow shows. Or, if you don't want to and those visions continue, take control of them. You could picture yourself getting purified, cleansed, or exorcised by some power. It's your imagination, remember that YOU can control it.