It's very difficult as seeing and hearing what you do will be very isolating as it is so real to you, even if you think it might not be real, doesn't stop it being terrifying.
As you've already spoken to your parents about feeling depressed, if you really don't feel that you can talk to them about this now, could you ask them if you could go to see your General Practitioner and go in alone. If you think it might be difficult to talk once you are there, could you write things down and give it to the doctor, or print off a copy of this and show it to them?
It's a lot to tell someone from the off and that might help you take a step towards getting some help.
It's not in the interests of them to take you to an institution unless, for example, there is an imminent serious risk of harm to yourself or others. Having such thoughts can be very different from acting on them, however unpleasant they are.
Your General Practitioner may also have information for you to take away about other places that you can find help and support when you need it, such as if you really need someone to talk to outside of medical office hours. They may also be able to help you in speaking to your parents if that would make it easier for you?
A first main step would be for them to get you to see someone where you can sit down in a safe space and talk through the things that are worrying you.
You are definitely not alone in what you are thinking and feeling and I hope that by talking about it, as you have taken a big step telling us, that you will get the help and support that you need.
I do wish you well with it all, it's not an easy thing to be dealing with and you don't have to deal with it alone.